[Hexagon] Use addAliasForDirective for data directives

Data directives such as .word, .half, .hword are currently parsed using 
HexagonAsmParser::ParseDirectiveValue which effectively duplicates logic from 
AsmParser::parseDirectiveValue. This patch deletes that duplicated logic in 
favour of using addAliasForDirective.

Differential Revision: https://reviews.llvm.org/D46999

llvm-svn: 332607