[X86] Use addAliasForDirective to support the .word directive (reland)

The X86 asm parser currently has custom parsing logic for .word. Rather than
use this custom logic, we can just use addAliasForDirective to enable the
reuse of AsmParser::parseDirectiveValue.

See also similar changes to Sparc (rL333078), AArch64 (rL333077), and Hexagon
(rL332607) backends.

Differential Revision: https://reviews.llvm.org/D47004

This is a fixed reland of rL336100. This should have been caught in 
pre-commit testing so apologies for the noise.

llvm-svn: 336104