Change SYMBOL_VALUE_ADDRESS to be an rvalue

Change SYMBOL_VALUE_ADDRESS to be an rvalue

This changes SYMBOL_VALUE_ADDRESS to be an rvalue.  The symbol readers
generally assign using this, so this also introduces
SET_SYMBOL_VALUE_ADDRESS and updates the readers.  Making this change
is useful in a subsequent patch, which redefined SYMBOL_VALUE_ADDRESS.

gdb/ChangeLog
2019-10-02  Tom Tromey  <tromey@adacore.com>

	* coffread.c (process_coff_symbol): Update.
	* dwarf2read.c (var_decode_location, new_symbol): Update.
	* mdebugread.c (parse_symbol): Update.
	* objfiles.c (relocate_one_symbol): Update.
	* stabsread.c (define_symbol, fix_common_block)
	(scan_file_globals): Update.
	* symtab.h (SYMBOL_VALUE_ADDRESS): Expand to an rvalue.
	(SET_SYMBOL_VALUE_ADDRESS): New macro.
	* xcoffread.c (process_xcoff_symbol): Update.