riscv: Fix up riscv_atomic_assign_expand_fenv [PR94950]

riscv: Fix up riscv_atomic_assign_expand_fenv [PR94950]

Similarly to the fixes on many other targets, riscv needs to use TARGET_EXPR
to avoid having the create_tmp_var_raw temporaries without proper DECL_CONTEXT
and not mentioned in local decls.

2020-05-06  Jakub Jelinek  <jakub@redhat.com>

	PR target/94950
	* config/riscv/riscv-builtins.c (riscv_atomic_assign_expand_fenv): Use
	TARGET_EXPR instead of MODIFY_EXPR for first assignment to old_flags.