crypto/rand: add syscall number fallbacks in seccomp_linux

[dougm]

seccomp_linux.go is only used for Go's own tests (commit 6446285), but the package is included when building programs with CGO_ENABLED=1.

When using an older glibc (e.g. 2.17), SYS_getrandom and SYS_seccomp are not defined, causing cgo compilation to fail.

Note that there are already several types and defines copied from linux headers into seccomp_linux.go to support compilation on older systems.

Updates #69536

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[dougm]

cc @FiloSottile

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