random.c (fill_random_bytes_syscall): use "__NR_" prefix on Linux

glibc still does not define the SYS_getrandom alias for
__NR_getrandom in the Linux kernel.  However, installing
up-to-date Linux kernel headers (linux-libc-dev >= 3.17 package
on Debian) will get the __NR_getrandom syscall number defined
properly without relying on glibc.

This allows users with a modern kernel+headers to use the getrandom
syscall without waiting on glibc support.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57307 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

random.c

@@ -505,7 +505,7 @@ fill_random_bytes_syscall(void *seed, size_t size, int unused)
     CryptGenRandom(prov, size, seed);
     return 0;
 }
-#elif defined __linux__ && defined SYS_getrandom
+#elif defined __linux__ && defined __NR_getrandom
 #include <linux/random.h>
 
 # ifndef GRND_NONBLOCK
@@ -522,7 +522,7 @@ fill_random_bytes_syscall(void *seed, size_t size, int need_secure)
 	if (!need_secure)
 	    flags = GRND_NONBLOCK;
 	errno = 0;
-	ret = syscall(SYS_getrandom, seed, size, flags);
+	ret = syscall(__NR_getrandom, seed, size, flags);
 	if (errno == ENOSYS) {
 	    ATOMIC_SET(try_syscall, 0);
 	    return -1;