x86; Add -mmanual-endbr and cf_check function attribute

x86; Add -mmanual-endbr and cf_check function attribute

Currently GCC inserts ENDBR instruction at entries of all non-static
functions, unless LTO compilation is used.  Marking all functions,
which are not called indirectly with nocf_check attribute, is not
ideal since 99% of functions in a program may be of this kind.

This patch adds -mmanual-endbr and cf_check function attribute.  They
can be used together with -fcf-protection such that ENDBR instruction
is inserted only at entries of functions with cf_check attribute.  It
can limit number of ENDBR instructions to reduce program size.

gcc/

	* config/i386/i386.c (rest_of_insert_endbranch): Insert ENDBR
	at the function entry only when -mmanual-endbr isn't used or
	there is cf_check function attribute.
	(ix86_attribute_table): Add cf_check.
	* config/i386/i386.opt: Add -mmanual-endbr.
	* doc/extend.texi: Document cf_check attribute.
	* doc/invoke.texi: Document -mmanual-endbr.

gcc/testsuite/

	* gcc.target/i386/cf_check-1.c: New test.
	* gcc.target/i386/cf_check-2.c: Likewise.
	* gcc.target/i386/cf_check-3.c: Likewise.
	* gcc.target/i386/cf_check-4.c: Likewise.
	* gcc.target/i386/cf_check-5.c: Likewise.

From-SVN: r267154