[X86] Support 'V' register operand modifier

This allows the register name to be printed without the leading '%'.
This can be used for emitting calls to the retpoline thunks from inline
asm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324645 91177308-0d34-0410-b5e6-96231b3b80d8