Re-land r361206 "[COFF] Store alignment in log2 form, NFC"

The previous patch lost the call to PowerOf2Ceil, which causes LLD to
crash when handling common symbols with a non-power-of-2 size. I tweaked
the existing common.test to make the bsspad16 common symbol be 15 bytes
to add coverage for this case.

llvm-svn: 361426