Added realloc implementation to GCs

When using the latest wasi-libc I experienced a
panic on an attempt to call realloc. My first attempt to
add it to arch_tinygowasm.go was obviously not good (PR #2194). So here
is another suggestion.

src/runtime/arch_tinygowasm.go

@@ -84,8 +84,7 @@ func libc_calloc(nmemb, size uintptr) unsafe.Pointer {
 
 //export realloc
 func libc_realloc(ptr unsafe.Pointer, size uintptr) unsafe.Pointer {
-	runtimePanic("unimplemented: realloc")
-	return nil
+	return realloc(ptr, size)
 }
 
 //export posix_memalign