Allow target to specify prefix for labels

Use the MCAsmInfo instead of the DataLayout, and allow
specifying a custom prefix for labels specifically. HSAIL
requires that labels begin with @, but global symbols with &.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223323 91177308-0d34-0410-b5e6-96231b3b80d8