[builtins] Fix building trunc*fhf2_test with GCC

[arichardson]

Summary

GCC does not support __fp16, use the TYPE_FP16 macro instead.

[compnerd]

GCC does support __fp16 though requires passing -mfp16-format. However, I think that it should be fine to use TYPE_FP16. Please do clean up the commit message though.

compiler-rt/test/builtins/Unit/truncdfhf2_test.c

int test__truncdfhf2(double a, uint16_t expected)
 
     if (ret){
         printf("error in test__truncdfhf2(%lf) = %#.4x, "
                "expected %#.4x\n", a, toRep16(x), expected);
     }
     return ret;
 }
 
-char assumption_1[sizeof(__fp16) * CHAR_BIT == 16] = {0};
+char assumption_1[sizeof(TYPE_FP16) * CHAR_BIT == 16] = {0};
 
 int main()
 {
     // qNaN
     if (test__truncdfhf2(makeQNaN64(),
                          UINT16_C(0x7e00)))
         return 1;
     // NaN

compiler-rt/test/builtins/Unit/truncsfhf2_test.c

int test__truncsfhf2(float a, uint16_t expected)
 
     if (ret){
         printf("error in test__truncsfhf2(%f) = %#.4x, "
                "expected %#.4x\n", a, toRep16(x), expected);
     }
     return ret;
 }
 
-char assumption_1[sizeof(__fp16) * CHAR_BIT == 16] = {0};
+char assumption_1[sizeof(TYPE_FP16) * CHAR_BIT == 16] = {0};
 
 int main()
 {
     // qNaN
     if (test__truncsfhf2(makeQNaN32(),
                          UINT16_C(0x7e00)))
         return 1;
     // NaN