sys-libs/glibc: enable IFUNC support only on whitelisted ARCHes, bug #641216

sys-libs/glibc: enable IFUNC support only on whitelisted ARCHes, bug #641216

We explicitly disable IFUNC support on the following targets:
    alpha/hppa/ia64/mips/m68k/nios2/riscv/sh
to workaround weak IFUNC detection on binutils/glibc side.

Otherwise at least on ia64 glibc generates IFUNC entries against
compat librt.so.1 symbols (to redirect them back to libc.so.6)
but linker does not produce correct relocations. As a result all
IFUNC-backed functions don't work.

Reported-by: Émeric Maschino
Bug: https://sourceware.org/PR22634
Closes: https://bugs.gentoo.org/641216
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>