aarch64: fix SVE ACLE check for bootstrap glibc builds

aarch64: fix SVE ACLE check for bootstrap glibc builds

arm_sve.h depends on stdint.h but that relies on libc headers unless
compiled in freestanding mode.  Without this change a bootstrap glibc
build (that uses a compiler without installed libc headers) failed with

checking for availability of SVE ACLE... In file included from [...]/arm_sve.h:28,
                 from conftest.c:1:
[...]/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. [...]