S390: Fix _FPU_SETCW/GETCW when compiling with Clang [BZ #30130]

The _FPU_SETCW and _FPU_GETCW macros are defined with inline assemblies.
They use the sfpc and efpc instructions, respectively.  But both contain
a spurious second operand that leads to a compile error with Clang.
Removing this operand works both with gcc/gas (since binutils 2.18) as
well as with clang/llvm.