configure: Check whether we can compile the s390-ccw bios with -msoft-float

configure: Check whether we can compile the s390-ccw bios with -msoft-float

The -msoft-float switch is not available in older versions of Clang.
Since we rely on the compiler to not generate floating point instructions
unexpectedly, we block those old compilers now via a test in the configure
script. Note that for some weird reasons, the Clang compiler only complains
about the missing soft-float support if no other flags are passed via
"-Wl,..." to the linker. So we have to use "compile_object" instead of
"compile_prog" for this check.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210525142032.156989-1-thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>