libgcc: aarch64: Configure check for __getauxval

libgcc: aarch64: Configure check for __getauxval

Add configure check for the __getauxval ABI symbol, which is always
available on aarch64 glibc, and may be available on other linux C
runtimes. For now only enabled on glibc, others have to override it

  target_configargs=libgcc_cv_have___getauxval=yes

This is deliberately obscure as it should be auto detected, ideally
via a feature test macro in unistd.h (link time detection is not
possible since the libc may not be installed at libgcc build time),
but currently there is no such feature test mechanism.

Without __getauxval, libgcc cannot do runtime CPU feature detection
and has to assume only the build time known features are available.

libgcc/ChangeLog:

	* config.in: Undef HAVE___GETAUXVAL.
	* configure: Regenerate.
	* configure.ac: Check for __getauxval.