60: Use a fixed-size type for the embedded bitcode header. r=ltratt a=vext01



Co-authored-by: Edd Barrett <vext01@gmail.com>

… >= 4G (llvm#151460)"

This reverts commit 600976f.

The test was crashing trying to access any element of the GPR struct.

(gdb) disas
Dump of assembler code for function _ZN7testing8internal11CmpHelperEQIyyEENS_15AssertionResultEPKcS4_RKT_RKT0_:
   0x00450afc <+0>:	push	{r4, r5, r6, r7, r8, r9, r10, r11, lr}
   0x00450b00 <+4>:	sub	sp, sp, rust-lang#60	; 0x3c
   0x00450b04 <+8>:	ldr	r5, [sp, rust-lang#96]	; 0x60
=> 0x00450b08 <+12>:	ldm	r3, {r4, r7}
   0x00450b0c <+16>:	ldm	r5, {r6, r9}
   0x00450b10 <+20>:	eor	r7, r7, r9
   0x00450b14 <+24>:	eor	r6, r4, r6
   0x00450b18 <+28>:	orrs	r7, r6, r7

(gdb) p/x r3
$3 = 0x3e300f6e

"However, load and store multiple instructions (LDM and STM) and load and store double-word (LDRD or STRD) must be aligned to at least a word boundary."

https://developer.arm.com/documentation/den0013/d/Porting/Alignment

>>> 0x3e300f6e % 4
2

Program received signal SIGBUS, Bus error.
0x00450b08 in testing::AssertionResult testing::internal::CmpHelperEQ<unsigned long long, unsigned long long>(char const*, char const*, unsigned long long const&, unsigned long long const&) ()

The struct is packed with 1 byte alignment, but it needs to start at an aligned address for us
to ldm from it. So I've done that with alignas.

Also fixed some compiler warnings in the test itself.