Since ld is Intel CET enabled on Intel CET enabled host, dlopen fails on
liblto_plugin.so if it isn't Intel CET enabled.  Add GCC_CET_HOST_FLAGS
to cet.m4, use it in libiberty and lto-plugin to always enable Intel
CET in liblto_plugin.so on Intel CET enabled host.

On Linux/x86 host, enable Intel CET by default if assembler and compiler
support Intel CET so that the generated liblto_plugin.so can be used on
both CET and non-CET machines.  It is an error to disable Intel CET in
liblto_plugin.so on Intel CET enabled host.

config/

	PR bootstrap/94739
	* cet.m4 (GCC_CET_HOST_FLAGS): New.

libiberty/

	PR bootstrap/94739
	* Makefile.in (COMPILE.c): Add @CET_HOST_FLAGS@.
	(configure_deps): Add $(srcdir)/../config/cet.m4 and
	$(srcdir)/../config/enable.m4.
	* aclocal.m4: Include ../config/cet.m4 and ../config/enable.m4.
	* configure.ac: Add GCC_CET_HOST_FLAGS(CET_HOST_FLAGS) and
	AC_SUBST(CET_HOST_FLAGS).
	* configure: Regenerated.

lto-plugin/

	PR bootstrap/94739
	* Makefile.am (AM_CFLAGS): Add $(CET_HOST_FLAGS).
	* configure.ac: Add GCC_CET_HOST_FLAGS(CET_HOST_FLAGS) and
	AC_SUBST(CET_HOST_FLAGS).
	* Makefile.in: Regenerated.
	* aclocal.m4: Likewise.
	* configure: Likewise.