rusticl/kernel: preserve fp16 denorms to fix vload/vstore_half

Fixes vload/vstore_half_rtp/rtn CL CTS tests

simple piglit cl-program-runner test application hitting this:

/*!
[config]
name: f32 mad with conversion from f16
clc_version_min: 10
#build_options: -cl-denorms-are-zero
dimensions: 1
[test]
name: test
kernel_name: test
local_size: 1 1 1
global_size: 1 0 0
arg_in: 0 uint 0xb8002001
arg_out: 1 buffer ushort[1] 0x8200
!*/

__attribute__((reqd_work_group_size(1, 1, 1)))
kernel void test(float in, half global* out)
{
	vstore_half_rtp(in, 0, out);
}