arch/riscv64: Support library call tracing

As PLT hooking is implemented, we can enable --libcall option on RISC-V.

  $ uftrace --force uname -m | head
  riscv64
  # DURATION     TID     FUNCTION
     4.800 us [  4503] | strrchr();
     5.766 ms [  4503] | setlocale();
     9.700 us [  4503] | bindtextdomain();
     5.600 us [  4503] | textdomain();
     2.700 us [  4503] | __cxa_atexit();
     7.000 us [  4503] | getopt_long();
     1.600 us [  4503] | getopt_long();
    13.500 us [  4503] | uname();

Signed-off-by: Namhyung Kim <namhyung@gmail.com>

uftrace.c

@@ -1389,10 +1389,7 @@ int main(int argc, char *argv[])
 	struct uftrace_opts opts = {
 		.mode = UFTRACE_MODE_INVALID,
 		.dirname = UFTRACE_DIR_NAME,
-#if !defined(__riscv)
-		/* FIXME: disable libcall until PLT hooking is implemented in riscv64. */
 		.libcall = true,
-#endif
 		.bufsize = SHMEM_BUFFER_SIZE,
 		.max_stack = OPT_RSTACK_DEFAULT,
 		.port = UFTRACE_RECV_PORT,