linux: Consolidate time implementation

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

Checked on 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>