libstdc++: Remove workaround for reserved init_priority warnings

Since r15-7511-g4e7f74225116e7 we can disable the warnings for using a
reserved priority using a diagnostic pragma. That means we no longer
need to put globals using that attribute into separate files that get
included.

This replaces the two uses of such separate files by moving the variable
definition into the source file and adding the diagnostic pragma.

libstdc++-v3/ChangeLog:

	* src/c++17/memory_resource.cc (default_res): Define here
	instead of including default_resource.h.
	* src/c++98/globals_io.cc (__ioinit): Define here instead of
	including ios_base_init.h.
	* src/c++17/default_resource.h: Removed.
	* src/c++98/ios_base_init.h: Removed.