math: Fix x86_64 build for -Os (BZ 33367)

math: Fix x86_64 build for -Os (BZ 33367)

The compiler might not inline the trunc function call for
USE_TRUNC_BUILTIN [1].

This patch adds an optimized __trunc/__truncf for x86 used
on modf ifunc variant to avoid the trunc libcall.

Checked on x86_64, x86_64-v2, x86_64-v3, and x86_64-v4. Used -O2 and
-Os options. Performed a full make check on x86_64 with both
 optimizations.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121861
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>