cmake: detect armv8 crc and crypto feature using CHECK_C_COMPILER_FLAG

cmake: detect armv8 crc and crypto feature using CHECK_C_COMPILER_FLAG

we are using GCC7 and up for C++17 support since mimic, and per
https://gcc.gnu.org/onlinedocs/gcc-4.9.1/gcc/AArch64-Options.html ,
GCC 4.9 and up should be able to support crc and crypto features if
these archs are enabled at GCC's configure-time. so we should always use
the -march for detecting the compiler's support instead of using the
inline assembly now.

GCC 4.8 is an ancient compiler, and per
https://gcc.gnu.org/gcc-4.8/changes.html, it was the the first GCC
release which offers AArch64 support. so we don't need to cater for
this GCC version. and we can trust GCC-7 and up.

Fixes: http://tracker.ceph.com/issues/17516
Signed-off-by: Kefu Chai <kchai@redhat.com>