Added the ParseInstruction() hook for target specific assembler directives so

Added the ParseInstruction() hook for target specific assembler directives so
that things like .word can be parsed as target specific.  Moved parsing .word
out of AsmParser.cpp into X86AsmParser.cpp as it is 2 bytes on X86 and 4 bytes
for other targets that support the .word directive.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81461 91177308-0d34-0410-b5e6-96231b3b80d8