MC: Introduce the ABS8 symbol modifier.

@ABS8 can be applied to symbols which appear as immediate operands to
instructions that have a 8-bit immediate form for that operand. It causes
the assembler to use the 8-bit form and an 8-bit relocation (e.g. R_386_8
or R_X86_64_8) for the symbol.

Differential Revision: https://reviews.llvm.org/D28688

llvm-svn: 293667