Linux kernel does this in fpregs_store() and fpregs_load(), so
qemu-user should do this as well.

Found by running valgrind's none/tests/s390x/test_sig.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220817123902.585623-1-iii@linux.ibm.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>