Also enable KexecFileLoad on linux/{arm64,riscv64} now that they have
SYS_KEXEC_FILE_LOAD.

Change-Id: I1c52d09a1a617794f2df697e4966c4c24a166611
Reviewed-on: https://go-review.googlesource.com/c/sys/+/164997
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>