crypto/rand: add syscall number fallbacks in seccomp_linux

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.

Author: dougm

src/crypto/internal/sysrand/internal/seccomp/seccomp_linux.go

@@ -49,6 +49,14 @@ struct seccomp_data {
 #define SECCOMP_RET_ALLOW 0x7fff0000U
 #define SECCOMP_SET_MODE_FILTER 1
 
+#ifndef SYS_getrandom
+#define SYS_getrandom -1
+#endif
+
+#ifndef SYS_seccomp
+#define SYS_seccomp -1
+#endif
+
 int disable_getrandom() {
     if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
         return 1;