fmv: Redirect to specific target

fmv: Redirect to specific target

Adds an optimisation in FMV to redirect to a specific target if possible.

A call is redirected to a specific target if both:
- the caller can always call the callee version
- and, it is possible to rule out all higher priority versions of the callee
  fmv set. That is estabilished either by the callee being the highest priority
  version, or each higher priority version of the callee implying that, were it
  resolved, a higher priority version of the caller would have been selected.

For this logic, introduces the new TARGET_OPTION_FUNCTIONS_B_RESOLVABLE_FROM_A
hook. Adds a full implementation for Aarch64, and a weaker default version
for other targets.

This allows the target to replace the previous optimisation as the new one is
able to cover the same case where two function sets implement the same versions.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_functions_b_resolvable_from_a): New
	function.
	(TARGET_OPTION_FUNCTIONS_B_RESOLVABLE_FROM_A): New define.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in: Add documentation for
	TARGET_OPTION_FUNCTIONS_B_RESOLVABLE_FROM_A.
	* multiple_target.cc (redirect_to_specific_clone): Add new optimisation
	logic.
	(ipa_target_clone): Remove check for TARGET_HAS_FMV_TARGET_ATTRIBUTE.
	* target.def: Document new hook..
	* attribs.cc: (functions_b_resolvable_from_a) New function.
	* attribs.h: (functions_b_resolvable_from_a) New function.

gcc/testsuite/ChangeLog:

	* g++.target/aarch64/fmv-selection1.C: New test.
	* g++.target/aarch64/fmv-selection2.C: New test.
	* g++.target/aarch64/fmv-selection3.C: New test.
	* g++.target/aarch64/fmv-selection4.C: New test.
	* g++.target/aarch64/fmv-selection5.C: New test.
	* g++.target/aarch64/fmv-selection6.C: New test.
	* g++.target/aarch64/fmv-selection7.C: New test.