[PATCH v5 2/2] Enable libmvec support for AArch64

[Joseph Myers]

This has broken configuring glibc for AArch64 in a bootstrap configuration 
(e.g. "compilers" build from build-many-glibcs.py).

checking for availability of SVE ACLE... In file included from /scratch/jmyers/glibc-bot/install/compilers/aarch64-linux-gnu/lib/gcc/aarch64-glibc-linux-gnu/14.0.0/include/arm_sve.h:28,
                 from conftest.c:1:
/scratch/jmyers/glibc-bot/install/compilers/aarch64-linux-gnu/lib/gcc/aarch64-glibc-linux-gnu/14.0.0/include/stdint.h:9:16: fatal error: stdint.h: No such file or directory
    9 | # include_next <stdint.h>
      |                ^~~~~~~~~~
compilation terminated.
configure: error: mathvec is enabled but compiler does not have SVE ACLE. Either use a compatible compiler or configure with --disable-mathvec (this results in incomplete ABI).

https://sourceware.org/pipermail/libc-testresults/2023q2/011207.html

It's possible using -ffreestanding in the configure test would avoid that 
dependence on having glibc headers already installed.

-- 
Joseph S. Myers
joseph@codesourcery.com