linux: Consolidate Linux gettimeofday

The IFUNC bypass to vDSO is used when USE_IFUNC_GETTIMEOFDAY is set.
Currently aarch64, powerpc*, and x86 defines it.  Otherwise the
generic implementation is used, which calls clock_gettime.

Checked on aarch64-linux-gnu, powerpc64le-linux-gnu,
powerpc64-linux-gnu, powerpc-linux-gnu-power4, x86_64-linux-gnu,
and i686-linux-gnu.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>