cgo: do a basic test that math functions work

They should, but we weren't testing this.
I discovered this while working on
https://github.com/tinygo-org/macos-minimal-sdk/pull/4 which will likely
make math.h not work anymore. So I wanted to make sure we have a test in
place before we update that dependency.

testdata/cgo/main.c

@@ -1,3 +1,4 @@
+#include <math.h>
 #include "main.h"
 
 int global = 3;
@@ -67,3 +68,7 @@ void unionSetData(short f0, short f1, short f2) {
 void arraydecay(int buf1[5], int buf2[3][8], int buf3[4][7][2]) {
 	// Do nothing.
 }
+
+double doSqrt(double x) {
+	return sqrt(x);
+}