revert "x86: Also pass -P to $(CPP) when processing i386-opc.tbl"

This reverts commit 384f368958f2a5bb083660e58e5f8a010e6ad429, which
broke i386-gen's emitting of diagnostics. As a replacement to address
the original issue of newer gcc no longer splicing lines when dropping
the line continuation backslashes, switch to using + as the line
continuation character, doing the line splicing in i386-gen.