Support the PGO build for binutils+gdb

Add the --enable-pgo-build[=lto] configure option.  When binutils+gdb
is not built together with GCC, --enable-pgo-build enables the PGO build:

1. First build with -fprofile-generate.
2. Use "make maybe-check-*" to generate profiling data and pass -i to make
to ignore errors when generating profiling data.
3. Use "make clean" to remove the previous build.
4. Rebuild with -fprofile-use.

With --enable-pgo-build=lto, -flto=jobserver -ffat-lto-objects are used
together with -fprofile-generate and -fprofile-use.  Add '+' to the command
line for recursive make to support -flto=jobserver -ffat-lto-objects.

NB: --enable-pgo-build=lto enables the PGO build with LTO while
--enable-lto enables LTO support in toolchain.

	PR binutils/26766
	* Makefile.tpl (BUILD_CFLAGS): New.
	(CFLAGS): Append $(BUILD_CFLAGS).
	(CXXFLAGS): Likewise.
	(PGO_BUILD_GEN_FLAGS_TO_PASS): New.
	(PGO_BUILD_TRAINING_CFLAGS): Likewise.
	(PGO_BUILD_TRAINING_CXXFLAGS): Likewise.
	(PGO_BUILD_TRAINING_FLAGS_TO_PASS): Likewise.
	(PGO_BUILD_TRAINING_MFLAGS): Likewise.
	(PGO_BUILD_USE_FLAGS_TO_PASS): Likewise.
	(PGO-TRAINING-TARGETS): Likewise.
	(PGO_BUILD_TRAINING): Likewise.
	(all): Add '+' to the command line for recursive make.  Support
	the PGO build.
	* configure.ac: Add --enable-pgo-build[=lto].
	AC_SUBST PGO_BUILD_GEN_CFLAGS, PGO_BUILD_USE_CFLAGS and
	PGO_BUILD_LTO_CFLAGS.  Enable the PGO build in Makefile.
	* Makefile.in: Regenerated.
	* configure: Likewise.