Test for linking for arm/size-optimization-ieee-[123].c

Test for linking for arm/size-optimization-ieee-[123].c

These tests require a target that supports arm soft-float.  The
problem is that the test checks for compile-time soft-float support,
but they may hit a problem when the linker complains that it can't
combine the testcase's object file with hard-float init files and
target system libraries.

I don't see that the tests actually require linking, and they could be
simplified to dg-do assemble, but I figured a link test for soft-float
support could be useful, so I added that, and adjusted the tests to
require it instead.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp
	(check_effective_target_arm_soft_ok_link): New.
	* gcc.target/arm/size-optimization-ieee-1.c: Use it.
	* gcc.target/arm/size-optimization-ieee-2.c: Likewise.
	* gcc.target/arm/size-optimization-ieee-3.c: Likewise.