[MSP430] Add support for .refsym directive

Introduces support for '.refsym' assembler directive.

From GCC docs (for MSP430):
'.refsym' - This directive instructs assembler to add an undefined reference
to the symbol following the directive. No relocation is created for this symbol;
it will exist purely for pulling in object files from archives.

Patch by Kristina Bessonova!

Differential Revision: https://reviews.llvm.org/D54618

llvm-svn: 347041