runtime: add runtime.rand function

This function is needed for Go 1.22, and is used from various packages
like math/rand.

When there is no random number generator available, it falls back to a
static sequence of numbers. I think this is fine, because as far as I
can see it is only used for non-cryptographic needs.

builder/musl.go

@@ -119,6 +119,7 @@ var Musl = Library{
 			"internal/syscall_ret.c",
 			"internal/vdso.c",
 			"legacy/*.c",
+			"linux/*.c",
 			"malloc/*.c",
 			"malloc/mallocng/*.c",
 			"mman/*.c",