arm: Extend the PR94780 fix to arm

Essentially the same fix as for x86.

2020-04-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/arm/arm-builtins.c (arm_atomic_assign_expand_fenv): Use
	TARGET_EXPR instead of MODIFY_EXPR for the first assignments to
	fenv_var and new_fenv_var.