include/fpu/softfloat: Fix compilation with Clang on s390x

include/fpu/softfloat: Fix compilation with Clang on s390x

Clang v7.0.1 does not like the __int128 variable type for inline
assembly on s390x:

In file included from fpu/softfloat.c:97:
include/fpu/softfloat-macros.h:647:9: error: inline asm error:
 This value type register class is not natively supported!
    asm("dlgr %0, %1" : "+r"(n) : "r"(d));
        ^

Disable this code part there now when compiling with Clang, so that
the generic code gets used instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>