Fix MIPS call-clobbered-*.c tests

Fix MIPS call-clobbered-*.c tests

Jeff pointed out that gcc.target/mips/call-clobbered-4.c started
failing after the function-abi series.  This is because IRA used
to treat partly call-clobbered registers as hard conflicts and
so wouldn't consider them for -fcaller-saves.  Now that we treat
call clobbers the same way regardless of where they come from,
we can use $f21 as a caller-save register.  This in turn means
that -Os is no longer a special case in call-clobbered-3.c.

(The new code is the same size as the old code.)

2019-10-02  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* gcc.target/mips/call-clobbered-3.c: Remove skip for -Os.
	* gcc.target/mips/call-clobbered-4.c: Delete.

From-SVN: r276456