Add .refsym to msp430 backend

Add .refsym to msp430 backend

* config/tc-msp430.c (msp430_refsym): New: ".refsym <symbol>"
* doc/c-msp430.texi (MSP430 Directives): Document it.

The purpose of this patch is to provide a way for one object file
to require the inclusion of another object, without having to
allocate space for a .word address reference.