correct target directive handling error handling

The target specific parser should return `false' if the target AsmParser handles
the directive, and `true' if the generic parser should handle the directive.
Many of the target specific directive handlers would `return Error' which does
not follow these semantics.  This change simply changes the target specific
routines to conform to the semantis of the ParseDirective correctly.

Conformance to the semantics improves diagnostics emitted for the invalid
directives.  X86 is taken as a sample to ensure that multiple diagnostics are
not presented for a single error.

llvm-svn: 199068