[MC] Add support for the .rva assembler directive for COFF targets

[MC] Add support for the .rva assembler directive for COFF targets

Even though gas doesn't document it, it has been supported there for
a very long time.

This produces the 32 bit relative virtual address (aka image relative
address) for a given symbol. ".rva foo" is essentially equal to
".long foo@imgrel".

Differential Revision: https://reviews.llvm.org/D49821

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338063 91177308-0d34-0410-b5e6-96231b3b80d8