Revert F16C series (MR 6774)
A couple of problems were discovered after this series was merged that cause breakage in different configurations:
(1) It seems that using -mf16c also enables AVX, leading to SIGILL on platforms that do not support AVX. (2) Since clang only warns about unknown flags, and as I understand it Meson's handling in cc.has_argument() is broken, the F16C code is wrongly enabled when clang is used, even for example on ARM, leading to a compilation error.
Closes: #3583 (closed)