options: Make -Ofast switch off -fsemantic-interposition

options: Make -Ofast switch off -fsemantic-interposition

Using -fno-semantic-interposition has been reported by various people
to bring about considerable speed up at the cost of strict compliance
to the ELF symbol interposition rules  See for example
https://fedoraproject.org/wiki/Changes/PythonNoSemanticInterpositionSpeedup

As such I believe it should be implied by our -Ofast optimization
level, not only so that benchmarks that can benefit run faster, but
also so that people looking at -Ofast documentation for options that
could speed their programs find it.

gcc/ChangeLog:

2021-11-12  Martin Jambor  <mjambor@suse.cz>

	* opts.c (default_options_table): Switch off
	flag_semantic_interposition at Ofast.
	* doc/invoke.texi (Optimize Options): Document that Ofast switches off
	-fsemantic-interposition.