AsmParser: add support for .end directive

The .end directive indicates the end of the file.  No further instructions are
processed after a .end directive is encountered.

One potential (glaringly obvious) optimisation that could be pursued here is to
extend MCAsmParser with a DiscardRemainder method to avoid processing lexemes to
the end of the file.  It was unclear at this point if that would be worth
adding, and could easily be added in a follow on change.

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>