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.

gas/doc/c-msp430.texi

@@ -263,6 +263,15 @@ command line option.
 @item .profiler
 This directive instructs assembler to add new profile entry to the object file.
 
+@cindex @code{refsym} directive, MSP 430
+@item .refsym
+This directive instructs assembler to add an undefined reference to
+the symbol following the directive.  The maximum symbol name length is
+1023 characters.  No relocation is created for this symbol; it will
+exist purely for pulling in object files from archives.  Note that
+this reloc is not sufficient to prevent garbage collection; use a
+KEEP() directive in the linker file to preserve such objects.
+
 @end table
 
 @node MSP430 Opcodes