build: Change conditional include and empty.mk to -include in Makefiles

build: Change conditional include and empty.mk to -include in Makefiles

GNU Make supports "-include" keyword to prevent warnings and errors due to
inclusion of non-existent files.  This patch changes gcc/ and libgcc/ to use
"-include" in place of the historical conditional inclusion and use of
empty.mk work-arounds.

gcc/ChangeLog

2020-06-23  David Edelsohn  <dje.gcc@gmail.com>

	* Makefile.in (LANG_MAKEFRAGS): Same.
	(tmake_file): Use -include.
	(xmake_file): Same.

libgcc/ChangeLog

2020-06-23  David Edelsohn  <dje.gcc@gmail.com>

	* Makefile.in: Remove uses of empty.mk. Use -include.
	* config/avr/t-avr: Use -include.
	* empty.mk: Delete.

libgcc/config/avr/libf7/ChangeLog

2020-06-23  David Edelsohn  <dje.gcc@gmail.com>

	* t-libf7: Same.