Add missing changes to Makefile.tpl

Update Makefile.tpl to add missing changes in

commit af019bfd
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Sat Jan 9 06:51:15 2021 -0800

    Support the PGO build for binutils+gdb

"autogen Makefile.def" showed no changes in Makefile.in.

	PR binutils/26766
	* Makefile.tpl (PGO_BUILD_TRAINING_FLAGS_TO_PASS): Add
	PGO_BUILD_TRAINING=yes.
	(PGO_BUILD_TRAINING_MFLAGS): New.
	(all): Pass $(PGO_BUILD_TRAINING_MFLAGS) to the PGO build.