[arm] fix bootstrap failure due to uninitialized warning

The Arm port is failing bootstrap because GCC is now warning about an
unitialized array.

The code is complex enough that I certainly can't be sure the compiler
is wrong, so perhaps the best fix here is just to memset the entire
array before use.

	* config/arm/arm.c (neon_valid_immediate): Clear bytes before use.

From-SVN: r277073