mips: Fix up mips_atomic_assign_expand_fenv [PR94780]

mips: Fix up mips_atomic_assign_expand_fenv [PR94780]

Commit message shamelessly copied from 1777beb6b129 by jakub:

This function, because it is sometimes called even outside of function
bodies, uses create_tmp_var_raw rather than create_tmp_var.  But in order
for that to work, when first referenced, the VAR_DECLs need to appear in a
TARGET_EXPR so that during gimplification the var gets the right
DECL_CONTEXT and is added to local decls.

gcc/

	PR target/94780
	* config/mips/mips.c (mips_atomic_assign_expand_fenv): Use
	  TARGET_EXPR instead of MODIFY_EXPR.

(cherry picked from commit 2065654435e3d97676366f82b939bc9273382dbe)