[PowerPC] Allow absolute expressions in relocations

The Linux kernel build uses absolute expressions suffixed with @lo/@ha
relocations. This currently doesn't work for DS/DQ form instructions and
there is no reason for it not to. It also works with GAS.
This patch allows this as long as the value is a multiple of 4/16
for DS/DQ form.

Differential revision: https://reviews.llvm.org/D115419