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/gc_none.go

@@ -12,6 +12,8 @@ import (
 
 func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer
 
+func realloc(ptr unsafe.Pointer, size uintptr) unsafe.Pointer
+
 func free(ptr unsafe.Pointer) {
 	// Nothing to free when nothing gets allocated.
 }