added architecture specific macros

1. Added __HIP_ARCH_GFXNUM__
2. Usage, -D__HIP_ARCH_GFX803__=1

Change-Id: I68b3a85d62cfab3a45d2b7a70cb3518ab2565236

hipamd/tests/src/deviceLib/hipTestHalf.cpp

@@ -25,6 +25,8 @@ THE SOFTWARE.
 #define HALF_SIZE 64*sizeof(__half)
 #define HALF2_SIZE 64*sizeof(__half2)
 
+#if __HIP_ARCH_GFX803__ > 0
+
 __global__ void __halfMath(hipLaunchParm lp, __half *A, __half *B, __half *C) {
   int tx = hipThreadIdx_x;
   __half a = A[tx];
@@ -56,6 +58,8 @@ __global__ void __half2Math(hipLaunchParm lp, __half2 *A, __half2 *B, __half2 *C
   c = __hmul2_sat(b, c);
 }
 
+#endif
+
 int main(){
   __half *A, *B, *C;
   hipMalloc(&A, HALF_SIZE);