Allow per-character control over what target assemblers allow in symbol

Allow per-character control over what target assemblers allow in symbol
names.  This also changes the default to allow all of "$_." in addition
to letters and numbers as symbol names.  If you don't want this, use
markCharUnacceptable to remove one of these or markCharAcceptable to add
to the set.  This corresponds with what GAS accepts by default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24291 91177308-0d34-0410-b5e6-96231b3b80d8