Removed the non-target independent AsmToken::Register enum constant

from MCAsmLexer.h in preparation of supporting other targets.  Changed the
X86AsmParser code to reflect this by removing AsmLexer::LexPercent and looking
for AsmToken::Percent when parsing in places that used AsmToken::Register.
Then changed X86ATTAsmParser::ParseRegister to parse out registers as an
AsmToken::Percent followed by an AsmToken::Identifier.

llvm-svn: 80929