After the customization handlers were added in
arm@bc8a58b
the optional no-fpregs attribute collided with the extern "C"
declaration on the handler - this was fixed by creating an extern "C"
block around it.

This cherry-picks changes relevant to llvmlibc-support from current
arm-software into the 22.x release branch:

12a4f2c [ATfE] Document _platform_debug_putc() (#723)
75faad2 [ATfE] Support command line options in LLVM libc (#705)
2ae2b6c [ATfE] Fix libc v7-R no-fpregs attribute build failure
(#720)
bc8a58b [ATfE] Provide LLVM libc handlers for hardware and runtime
setup (#706)
852e751 [ATfE] Provide debug output handler in LLVM libc crt0
9be5d72 (origin/add_unhosted_exit, add_unhosted_exit) Simplified
implementation of exit to avoid issues with privileged assembly
instructions
983f37c [ATfE] Provide nohost init and exit in llvmlibc startup
code
6859f67 [ATfE] Replace call to abort with __llvm_libc_exit in libc
startup code (#678)
a284205 (origin/update_exit_comment, update_exit_comment) [ATfE]
Update comment about handling cleanup for exit()
fb865b8 (origin/remove_abort_redefinition,
remove_abort_redefinition) [ATfE] Replace call to abort with
__llvm_libc_exit in libc startup code
28c43df (origin/use_sys_readc_for_stdin) Merge branch
'arm-software' into use_sys_readc_for_stdin
53e062f (use_sys_readc_for_stdin) Added a comment with rationale
and TODO
bbf19b6 (origin/add_semihosting_abort, add_semihosting_abort) Mark
internal semihosting function as static
52d1d90 Provide semihosting_call_exit() and add TODO for exit()
99c608d Replace exit() with direct call to __llvm_libc_exit()
07fe14b [ATfE] Use semihosting SYS_READC for stdin with llvm libc
ab08655 [ATfE] Redirect libc abort() to semihosting exit()