Add basic ABI tests for _Float16 type

Just very trivial argument passing, but it can still go wrong in some cases.
I've seen the compiler generating instructions invalid for the target
architecture, for example.

Author: TNorthover

SingleSource/Regression/C/float16-smoke.c

@@ -0,0 +1,33 @@
+#include <stdio.h>
+
+
+#ifdef __FLT16_DIG__
+
+typedef _Float16 fp16_t;
+__attribute__((noinline))
+void printArg(fp16_t a0) {
+  printf("printArg: %a\n", (double)a0);
+}
+
+fp16_t g_fp16 = 1.0;
+
+int main(int argc, char** argv) {
+  printArg((fp16_t)0.0);
+  printArg((fp16_t)0x1p0);
+  printArg((fp16_t)-0x1p-8);
+  printArg(g_fp16 + (fp16_t)0x1p0);
+
+  return 0;
+}
+
+#else
+
+int main() {
+  printf("printArg: 0x0p+0\n");
+  printf("printArg: 0x1p+0\n");
+  printf("printArg: -0x1p-8\n");
+  printf("printArg: 0x1p+1\n");
+  return 0;
+}
+
+#endif

SingleSource/Regression/C/float16-smoke.reference_output

@@ -0,0 +1,5 @@
+printArg: 0x0p+0
+printArg: 0x1p+0
+printArg: -0x1p-8
+printArg: 0x1p+1
+exit 0