MIPS: Add options to control branch ISA checks
    
    Complement commit 9d862524f6ae ("MIPS: Verify the ISA mode and alignment
    of branch and jump targets") and add GAS and LD options to control the
    checks for invalid branches between ISA modes introduced there, to help
    with some handwritten code lacking `.insn' annotation for labels used as
    branch targets and code produced by older versions of GCC which suffers
    from the issue with branches to code that has been optimized away,
    addressed with GCC commit 242424 ("MIPS/GCC: Mark trailing labels with
    `.insn'"), <https://gcc.gnu.org/ml/gcc-patches/2016-11/msg01061.html>.
    
    	bfd/
    	* elfxx-mips.h (_bfd_mips_elf_insn32): Rename prototype to...
    	(_bfd_mips_elf_linker_flags): ... this.  Add another parameter.
    	* elfxx-mips.c (mips_elf_link_hash_table): Add
    	`ignore_branch_isa' member.
    	(mips_elf_perform_relocation): Do not treat an ISA mode mismatch
    	in branch relocation calculation as an error if
    	`ignore_branch_isa' has been set.
    	(_bfd_mips_elf_insn32): Rename to...
    	(_bfd_mips_elf_linker_flags): ... this.  Rename the `on'
    	parameter to `insn32' and add an `ignore_branch_isa' parameter.
    	Handle the new parameter.
    
    	gas/
    	* config/tc-mips.c (mips_ignore_branch_isa): New variable.
    	(options): Add OPTION_IGNORE_BRANCH_ISA and
    	OPTION_NO_IGNORE_BRANCH_ISA enum values.
    	(md_longopts): Add "mignore-branch-isa" and
    	"mno-ignore-branch-isa" options.
    	(md_parse_option): Handle OPTION_IGNORE_BRANCH_ISA and
    	OPTION_NO_IGNORE_BRANCH_ISA.
    	(fix_bad_cross_mode_branch_p): Return FALSE if
    	`mips_ignore_branch_isa' has been set.
    	(md_show_usage): Add `-mignore-branch-isa' and
    	`-mno-ignore-branch-isa'.
    
    	* doc/as.texinfo (Target MIPS options): Add
    	`-mignore-branch-isa' and `-mno-ignore-branch-isa' options.
    	(-mignore-branch-isa, -mno-ignore-branch-isa): New options.
    	* doc/c-mips.texi (MIPS Options): Add `-mignore-branch-isa' and
    	`-mno-ignore-branch-isa' options.
    
    	* testsuite/gas/mips/branch-local-ignore-2.d: New test.
    	* testsuite/gas/mips/branch-local-ignore-3.d: New test.
    	* testsuite/gas/mips/branch-local-ignore-n32-2.d: New test.
    	* testsuite/gas/mips/branch-local-ignore-n32-3.d: New test.
    	* testsuite/gas/mips/branch-local-ignore-n64-2.d: New test.
    	* testsuite/gas/mips/branch-local-ignore-n64-3.d: New test.
    	* testsuite/gas/mips/mips.exp: Run the new tests.
    
    	ld/
    	* emultempl/mipself.em (ignore_branch_isa): New variable.
    	(mips_create_output_section_statements): Rename
    	`_bfd_mips_elf_insn32' called to `_bfd_mips_elf_linker_flags',
    	add `ignore_branch_isa' argument.
    	(PARSE_AND_LIST_PROLOGUE): Add OPTION_IGNORE_BRANCH_ISA and
    	OPTION_NO_IGNORE_BRANCH_ISA enum values.
    	(PARSE_AND_LIST_LONGOPTS): Add "ignore-branch-isa" and
    	"no-ignore-branch-isa" options.
    	(PARSE_AND_LIST_OPTIONS): Add `--ignore-branch-isa' and
    	`--no-ignore-branch-isa'.
    	(PARSE_AND_LIST_ARGS_CASES): Handle OPTION_IGNORE_BRANCH_ISA and
    	OPTION_NO_IGNORE_BRANCH_ISA.
    
    	* ld.texinfo (Options specific to MIPS targets): Add
    	`--ignore-branch-isa' and `--no-ignore-branch-isa' options.
    	(ld and the MIPS family): Likewise.
    
    	* testsuite/ld-mips-elf/bal-jalx-pic-ignore.d: New test.
    	* testsuite/ld-mips-elf/bal-jalx-pic-ignore-n32.d: New test.
    	* testsuite/ld-mips-elf/bal-jalx-pic-ignore-n64.d: New test.
    	* testsuite/ld-mips-elf/unaligned-branch-ignore-2.d: New test.
    	* testsuite/ld-mips-elf/unaligned-branch-ignore-r6-1: New test.
    	* testsuite/ld-mips-elf/unaligned-branch-ignore-mips16: New
    	test.
    	* testsuite/ld-mips-elf/unaligned-branch-ignore-micromips: New
    	test.
    	* testsuite/ld-mips-elf/mips-elf.exp: Run the new tests.
    
    (cherry picked from commit 8b10b0b3e100c25322a083248c7a18bf5a1f3527)