[AArch64, 6/6] Enable BTI: Add configure option.

[AArch64, 6/6] Enable BTI: Add configure option.

This patch is part of a series that enables ARMv8.5-A in GCC and
adds Branch Target Identification Mechanism.

This patch is adding a new configure option for enabling BTI and
Return Address Signing by default.

*** gcc/ChangeLog ***

2018-01-09  Sudakshina Das  <sudi.das@arm.com>

	* config/aarch64/aarch64.c (aarch64_override_options): Add case to
	check configure option to set BTI and Return Address Signing.
	* configure.ac: Add --enable-standard-branch-protection and
	--disable-standard-branch-protection.
	* configure: Regenerated.
	* doc/install.texi: Document the same.

*** gcc/testsuite/ChangeLog ***

2018-01-09  Sudakshina Das  <sudi.das@arm.com>

	* gcc.target/aarch64/bti-1.c: Update test to not add command line
	option when configure with bti.
	* gcc.target/aarch64/bti-2.c: Likewise.
	* lib/target-supports.exp
	(check_effective_target_default_branch_protection):
	Add configure check for --enable-standard-branch-protection.

From-SVN: r267770