Abstract ifunc support into a define
Summary:
There are platforms other than clang that don't support ifuncs. (The one
I'm concerned about is ARM.) I changed the ifdef __clang__ around the
ifunc attributes to be more abstract, so we can can pass in this flag on
the command line, or use autoconf to detect it.
Test Plan:
fbmake runtests. Manually define HAVE_IFUNC 0 and make sure the
popcount() and popcountll() functions get compiled as calls to
popcount_builtin.
Run autoreconf, ./configure, make sure the feature gets detected
properly by looking at config.h.
Reviewed By: andrewjcg@fb.com
FB internal diff:
D712192