gdb: make macro_stringify return a gdb::unique_xmalloc_ptr<char>

The change to macro_stringify is straightforward.  This allows removing
the manual memory management in fixup_definition.

gdb/ChangeLog:

	* macroexp.h (macro_stringify): Return
	gdb::unique_xmalloc_ptr<char>.
	* macroexp.c (macro_stringify): Likewise.
	* macrotab.c (fixup_definition): Update.

Change-Id: Id7db8988bdbd569dd51c4f4655b00eb26db277cb