[compiler-rt][libunwind] Check elf_aux_info return value (#228190)

elf_aux_info() returns an errno value (0 on success) rather than -1
with errno set. Check the result and treat a failure as "no features
reported" instead of silently ignoring the error.

In compiler-rt, hwcap had no initializer, so a failed call left it
uninitialized. The libunwind users already started from zero, so the
change there only makes the error handling explicit.

GitOrigin-RevId: 7710aeb5bc1f7ad54dcc4b2a9826e0c67b60d5db