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>

include/fpu/softfloat-macros.h

@@ -641,7 +641,7 @@ static inline uint64_t udiv_qrnnd(uint64_t *r, uint64_t n1,
     uint64_t q;
     asm("divq %4" : "=a"(q), "=d"(*r) : "0"(n0), "1"(n1), "rm"(d));
     return q;
-#elif defined(__s390x__)
+#elif defined(__s390x__) && !defined(__clang__)
     /* Need to use a TImode type to get an even register pair for DLGR.  */
     unsigned __int128 n = (unsigned __int128)n1 << 64 | n0;
     asm("dlgr %0, %1" : "+r"(n) : "r"(d));