testsuite: Add target/xfail argument to check-function-bodies

check-function-bodies allows individual function tests to be
annotated with target/xfail selectors, but sometimes it's
useful to have the same selector for all functions.

2020-01-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* doc/sourcebuild.texi (check-function-bodies): Add an
	optional target/xfail selector.

gcc/testsuite/
	* lib/scanasm.exp (check-function-bodies): Add an optional
	target/xfail selector.