internal/syscall/unix: eliminate non-trivial randomTrap initializer

While here, enable getrandom on arm64 too (using the value found in
include/uapi/asm-generic/unistd.h, which seems to match up with other
GOARCH=arm64 syscall numbers).

Updates #10848.

Change-Id: I5ab36ccf6ee8d5cc6f0e1a61d09f0da7410288b9
Reviewed-on: https://go-review.googlesource.com/16662


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>