Fix parsing of .symver directive on ARM

Fix parsing of .symver directive on ARM

ARM assembly syntax uses @ for a comment, execpt for the second
parameter of the .symver directive which requires @ as part of the
symbol name. This commit fixes the parsing of this directive by
adding a special case for ARM for this one argumnet.

To make the change we had to move the AllowAtInIdentifier variable
to the MCAsmLexer interface (from AsmLexer) and expose a setter for
the value.  The ELFAsmParser then toggles this value when parsing
the second argument to the .symver directive for a target that
uses @ as a comment symbol

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199339 91177308-0d34-0410-b5e6-96231b3b80d8