Enable building libgcc with CET options.

Enable building libgcc with CET options.

Enable building libgcc with CET options by default on Linux/x86 if
binutils supports CET v2.0.  It can be disabled with --disable-cet.
It is an error to configure GCC with --enable-cet if bintuiils
doesn't support CET v2.0.

ENDBR instruction is added to __morestack_large_model since it is
called indirectly.

2017-11-17  Igor Tsimbalist  <igor.v.tsimbalist@intel.com>

config/
	* cet.m4: New file.

gcc/
	* config.gcc (extra_headers): Add cet.h for x86 targets.
	* config/i386/cet.h: New file.
	* doc/install.texi: Add --enable-cet/--disable-cet.

libgcc/
	* Makefile.in (configure_deps): Add $(srcdir)/../config/cet.m4.
	(CET_FLAGS): New.
	* config/i386/morestack.S: Include <cet.h>.
	(__morestack_large_model): Add _CET_ENDBR at function entrance.
	* config/i386/resms64.h: Include <cet.h>.
	* config/i386/resms64f.h: Likewise.
	* config/i386/resms64fx.h: Likewise.
	* config/i386/resms64x.h: Likewise.
	* config/i386/savms64.h: Likewise.
	* config/i386/savms64f.h: Likewise.
	* config/i386/t-linux (HOST_LIBGCC2_CFLAGS): Add $(CET_FLAGS).
	(CRTSTUFF_T_CFLAGS): Likewise.
	* configure.ac: Include ../config/cet.m4.
	Set and substitute CET_FLAGS.
	* configure: Regenerated.

From-SVN: r254868