libiberty: Fix build with GCC < 7

libiberty: Fix build with GCC < 7

Tobias reported on IRC that the linker fails to build with GCC 4.8.5.
In configure I've tried to use everything actually used in the sha1.c
x86 hw implementation, but unfortunately I forgot about implicit function
declarations.  GCC before 7 did have <cpuid.h> header and bit_SHA define
and __get_cpuid function defined inline, but it didn't define
__get_cpuid_count, which compiled fine (and the configure test is
intentionally compile time only) due to implicit function declaration,
but then failed to link when linking the linker, because
__get_cpuid_count wasn't defined anywhere.

The following patch fixes that by using what autoconf uses in AC_CHECK_DECL
to make sure the functions are declared.

2023-12-05  Jakub Jelinek  <jakub@redhat.com>

	* configure.ac (HAVE_X86_SHA1_HW_SUPPORT): Verify __get_cpuid and
	__get_cpuid_count are not implicitly declared.
	* configure: Regenerated.