rs6000: Fix rs6000_atomic_assign_expand_fenv [PR94826]

rs6000: Fix rs6000_atomic_assign_expand_fenv [PR94826]

This is the rs6000 version of the earlier committed x86, aarch64 and arm
fixes, as create_tmp_var_raw is used because the C FE can call this outside
of function context, we need to make sure the first references to those
VAR_DECLs are through a TARGET_EXPR, so that it gets gimple_add_tmp_var
marked in whatever function it gets expanded in.  Without that DECL_CONTEXT
is NULL and the vars aren't added as local decls of the containing function.

2020-04-29  Jakub Jelinek  <jakub@redhat.com>

	PR target/94826
	* config/rs6000/rs6000.c (rs6000_atomic_assign_expand_fenv): Use
	TARGET_EXPR instead of MODIFY_EXPR for first assignment to
	fenv_var, fenv_clear and old_fenv variables.  For fenv_addr
	take address of TARGET_EXPR of fenv_var with void_node initializer.
	Formatting fixes.

(cherry picked from commit c7137fcc7cbc1f1f14f9fed75adcc6bd8f1d418c)