[MC] Skip lower-case integer suffixes

`mov x0, 1024u` is permitted in binutils but rejected by the integrated
assembler. Support the case. This is especially important when using the C
pre-processor with the assembler: some shared code between C and assembler may
use lower-cased suffices.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D128871