wasm: add support for the crypto/rand package

This is done via wasi-libc and the WASI interface, for ease of
maintenance (only one implementation for both WASI and JS/browsers).

src/crypto/rand/rand_getentropy.go

@@ -1,4 +1,4 @@
-// +build darwin freebsd wasi
+// +build darwin freebsd tinygo.wasm
 
 // This implementation of crypto/rand uses the getentropy system call (available
 // on both MacOS and WASI) to generate random numbers.