x86: Fix x86_64 build failure with -Os (BZ 33367)

x86: Fix x86_64 build failure with -Os (BZ 33367)

The 13cfd77bf5 change broke the b5d88fa6c3 fix by removing the symbol
to __symbol redirections.  Although it works for -O2 with both gcc
and clang, with -Os without the redirection, the libcall might still
be issued.

This patch reinstates the b5d88fa6c3 fix, with a modification that
allows each ifunc variant to control which trunc to issue.  This is
required for clang, which defines HAVE_X86_INLINE_TRUNC to 1 (meaning
that trunc will always be lowered to the instruction on -Os).

Checked on x86_64-linux-gnu with -O2 and -Os with gcc-15 and clang-18.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>