Extend MCAsmLexer so that it can peek forward several tokens

This commit adds a virtual `peekTokens()` function to `MCAsmLexer`
which can peek forward an arbitrary number of tokens.

It also makes the `peekTok()` method call `peekTokens()` method, but
only requesting one token.

The idea is to better support targets which more more ambiguous
assembly syntaxes.

Patch by Dylan McKay!

llvm-svn: 245221