Add .data and .bss refsym symbols

Add .data and .bss refsym symbols

For each object, if it has a nonempty .data or .bss section,
emit a symbol that could cause the startup code to selectively
link in the code to initialize those sections.

* config/tc-msp430.c (msp430_section): Always flag data sections,
regardless of -md.
(msp430_frob_section): New.  Make sure all sections are noticed if
they have content.
(msp430_lcomm): New.  Flag bss if .lcomm is seen.
(msp430_comm): New.  Likewise.
(md_pseudo_table): Add them.
* config/tc-msp430.h (msp430_frob_section): Declare.
(tc_frob_section): Define.