tsan: Add param to disable func-entry-exit instrumentation

tsan: Add param to disable func-entry-exit instrumentation

Adds param tsan-instrument-func-entry-exit, which controls if
__tsan_func_{entry,exit} calls should be emitted or not. The default
behaviour is to emit the calls.

This may be required by alternative race detection runtimes. One such
runtime is the Kernel Concurrency Sanitizer (KCSAN):

	https://github.com/google/ktsan/wiki/KCSAN

After this change, GCC should satisfy all requirements for KCSAN:

	https://lore.kernel.org/lkml/20200515150338.190344-7-elver@google.com/

gcc/ChangeLog:

	* gimplify.c (gimplify_function_tree): Optimize and do not emit
	IFN_TSAN_FUNC_EXIT in a finally block if we do not need it.
	* params.opt: Add --param=tsan-instrument-func-entry-exit=.
	* tsan.c (instrument_memory_accesses): Make
	fentry_exit_instrument bool depend on new param.

gcc/testsuite/ChangeLog:

	* c-c++-common/tsan/func_entry_exit.c: New test.
	* c-c++-common/tsan/func_entry_exit_disabled.c: New test.