Add a get_random_bytes() syscall to replace the SRPC-based implementation

tests/random/nacl.scons

Index: tests/random/nacl.scons
diff --git a/tests/random/nacl.scons b/tests/random/nacl.scons
index d40298fcc865f3e53e04ef0cdf5602bf293411c1..f7f31aaaedf13b4fd6e200646cbe327fbc82a252 100644
--- a/tests/random/nacl.scons
+++ b/tests/random/nacl.scons
@@ -5,19 +5,13 @@
 
 Import('env')
 
-# When testing the private-only nacl_secure_random(), we must link in
-# a lot of dependencies and initialize libsrpc manually.
-# TODO(mseaborn): Clean this up by removing irt_random.c's use of SRPC.
-env.Append(CPPDEFINES=[['TESTS_USE_IRT',
-                        str(int(env.Bit('tests_use_irt')))]])
 libs = []
 if not env.Bit('tests_use_irt'):
   if env.Bit('nacl_glibc'):
     # libnacl_random_private depends on functions in irt_blockhook.c,
     # which we don't have when building against nacl-glibc.
     Return()
-  libs += ['libnacl_random_private', '${PTHREAD_LIBS}', 'srpc',
-           'platform', 'imc_syscalls']
+  libs += ['libnacl_random_private']
 
 nexe = env.ComponentProgram(
     env.ProgramNameForNmf('random_test'), 'random_test.c',