asan considers that memcmp(p, q, N) is permitted to access N bytes at each
of p and q, even for values of p and q that have a difference earlier.
Accessing additional values is frequently done in practice, reading 4 or
more bytes from each input at a time for efficiency, so when completing
"non_exist<TAB>" in the repl, this causes a diagnostic:

    ==16938==ERROR: AddressSanitizer: global-buffer-overflow on
    address 0x555555cd8dc8 at pc 0x7ffff726457b bp 0x7fffffffda20 sp 0x7fff
    READ of size 9 at 0x555555cd8dc8 thread T0
        #0 0x7ffff726457a  (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xb857a)
        #1 0x555555b0e82a in mp_repl_autocomplete ../../py/repl.c:301
        #2 0x555555c89585 in readline_process_char ../../lib/mp-readline/re
        #3 0x555555c8ac6e in readline ../../lib/mp-readline/readline.c:513
        #4 0x555555b8dcbd in do_repl /home/jepler/src/micropython/ports/uni
        #5 0x555555b90859 in main_ /home/jepler/src/micropython/ports/unix/
        #6 0x555555b90a3a in main /home/jepler/src/micropython/ports/unix/m
        #7 0x7ffff619a09a in __libc_start_main ../csu/libc-start.c:308
        #8 0x55555595fd69 in _start (/home/jepler/src/micropython/ports/uni

    0x555555cd8dc8 is located 0 bytes to the right of global variable
    'import_str' defined in '../../py/repl.c:285:23' (0x555555cd8dc0) of
    size 8
      'import_str' is ascii string 'import '

Signed-off-by: Jeff Epler <jepler@gmail.com>

Although the original motivation given for the workaround[1] is correct,
nlr.o and nlrthumb.o are linked with a small enough distance that the
problem does not occur, and the workaround isn't necessary. The distance
between the b instruction and its target (nlr_push_tail) is just 64
bytes[2], well within the ±2046 byte range addressable by an
unconditional branch instruction in Thumb mode.

The workaround induces a relocation in the text section (textrel), which
isn't supported everywhere, notably not on musl-libc[3], where it causes
a crash on start-up. With the workaround removed, micropython works on an
ARMv5T Linux system built with musl-libc.

This commit changes nlrthumb.c to use a direct jump by default, but
leaves the long jump workaround as an option for those cases where it's
actually needed.

[1]: commit dd376a2

Author: Damien George <damien.p.george@gmail.com>
Date:   Fri Sep 1 15:25:29 2017 +1000

    py/nlrthumb: Get working again on standard Thumb arch (ie not Thumb2).

    "b" on Thumb might not be long enough for the jump to nlr_push_tail so
    it must be done indirectly.

[2]: Excerpt from objdump -d micropython:

000095c4 <nlr_push_tail>:
    95c4:       b510            push    {r4, lr}
    95c6:       0004            movs    r4, r0
    95c8:       f02d fd42       bl      37050 <mp_thread_get_state>
    95cc:       6943            ldr     r3, [r0, #20]
    95ce:       6023            str     r3, [r4, #0]
    95d0:       6144            str     r4, [r0, #20]
    95d2:       2000            movs    r0, #0
    95d4:       bd10            pop     {r4, pc}

000095d6 <nlr_pop>:
    95d6:       b510            push    {r4, lr}
    95d8:       f02d fd3a       bl      37050 <mp_thread_get_state>
    95dc:       6943            ldr     r3, [r0, #20]
    95de:       681b            ldr     r3, [r3, #0]
    95e0:       6143            str     r3, [r0, #20]
    95e2:       bd10            pop     {r4, pc}

000095e4 <nlr_push>:
    95e4:       60c4            str     r4, [r0, #12]
    95e6:       6105            str     r5, [r0, #16]
    95e8:       6146            str     r6, [r0, #20]
    95ea:       6187            str     r7, [r0, micropython#24]
    95ec:       4641            mov     r1, r8
    95ee:       61c1            str     r1, [r0, micropython#28]
    95f0:       4649            mov     r1, r9
    95f2:       6201            str     r1, [r0, micropython#32]
    95f4:       4651            mov     r1, sl
    95f6:       6241            str     r1, [r0, micropython#36]   @ 0x24
    95f8:       4659            mov     r1, fp
    95fa:       6281            str     r1, [r0, micropython#40]   @ 0x28
    95fc:       4669            mov     r1, sp
    95fe:       62c1            str     r1, [r0, micropython#44]   @ 0x2c
    9600:       4671            mov     r1, lr
    9602:       6081            str     r1, [r0, #8]
    9604:       e7de            b.n     95c4 <nlr_push_tail>

[3]: https://www.openwall.com/lists/musl/2020/09/25/4

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>