[X86] Use addAliasForDirective to support the .word directive

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

llvm-svn: 336100