Integrated assembler incorrectly lexes ARM-style comments

The integrated assembler fails to properly lex arm comments when
they are adjacent to an identifier in the input stream. The reason
is that the arm comment symbol '@' is also used as symbol variant in
other assembly languages so when lexing an identifier it allows the
'@' symbol as part of the identifier.

Example:
  $ cat comment.s
  foo:
    add r0, r0@got to parse this as a comment

  $ llvm-mc -triple armv7 comment.s
  comment.s:4:18: error: unexpected token in argument list
    add r0, r0@got to parse this as a comment
                   ^
This should be parsed as correctly as `add r0, r0`.

This commit modifes the assembly lexer to not include the '@' symbol
in identifiers when lexing for targets that use '@' for comments.