s390: Fix up s390_atomic_assign_expand_fenv

s390: Fix up s390_atomic_assign_expand_fenv

The following patch fixes
-FAIL: gcc.dg/pr94780.c (internal compiler error)
-FAIL: gcc.dg/pr94780.c (test for excess errors)
-FAIL: gcc.dg/pr94842.c (internal compiler error)
-FAIL: gcc.dg/pr94842.c (test for excess errors)
on s390x-linux.  The fix is essentially the same as has been applied to many
other targets (i386, aarch64, arm, rs6000, alpha, riscv).

2020-10-01  Jakub Jelinek  <jakub@redhat.com>

	* config/s390/s390.c (s390_atomic_assign_expand_fenv): Use
	TARGET_EXPR instead of MODIFY_EXPR for the first assignments to
	fenv_var and old_fpc.  Formatting fixes.