MC: Add AsmLexer::BigNum token for integers greater than 64 bits

This will be needed for .octa support, but we don't want to just use the
existing AsmLexer::Integer for it and then have to litter all its users
with explicit checks for the size, and make them use the new get APIntVal()
method.

So let the lexer produce an AsmLexer::Integer as before for numbers which
are small enough — which appears to cover what was previously a nasty
special case handling of numbers which don't fit in int64_t but *do* fit
in uint64_t.

Where the number is too large even for that, produce an AsmLexer::BigNum
instead. We do nothing with these except complain about them for now,
but that will be changed shortly...

Based on a patch from PaX Team <pageexec@freemail.hu>
MC: Add support for .octa

This is a minimal implementation which accepts only constants rather than
full expressions, but that should be perfectly sufficient for all known
users for now.

Patch from PaX Team <pageexec@freemail.hu>