Add STRINGIFY to gdb/common/preprocessor.h

Add STRINGIFY to gdb/common/preprocessor.h

We have several copies of this common idiom under gdb/ currently.
This commit moves them / factors them out to gdb/common/preprocessor.h.

gdb/ChangeLog:
2017-06-22  Pedro Alves  <palves@redhat.com>

	* common/agent.h: Include "common/preprocessor.h".
	(STRINGIZE_1, STRINGIZE): Delete.
	(IPA_SYM): Use STRINGIFY instead.
	* common/preprocessor.h (STRINGIFY_1, STRINGIFY): New.
	* compile/compile-c-support.c: Include "common/preprocessor.h".
	(STR, STRINGIFY): Delete.
	* ia64-libunwind-tdep.c: Include "common/preprocessor.h".
	(STRINGIFY2, STRINGIFY): Delete.