wc: port to kernels that disable XSAVE YMM

wc: port to kernels that disable XSAVE YMM

Problem reported by Dave Hansen <https://bugs.gnu.org/64058>.
Apply similar change to cksum and pclmul, too.
* NEWS: Mention wc fix.
* configure.ac (cpuid_exists, get_cpuid_count_exists):
Remove.  All uses removed, since we no longer use __get_cpuid or
__get_cpuid_count.
(pclmul_intrinsic_exists, avx2_intrinsic_exists): Set to no if
__builtin_cpu_supports calls cannot be compiled.
(HAVE_PCLMUL_INTRINSIC, HAVE_AVX2_INTRINSIC): Remove; unused.
Simplify surrounding code because of this.
* src/cksum.c (pclmul_supported):
* src/wc.c (avx2_supported):
Use __builtin_cpu_supports instead of doing it by hand.
Simplify surrounding code because of this.