RISC-V: Add _LITE_EXIT in crt0.S.

This patch adds _LITE_EXIT in crt0.S to enable "lite exit" technique in
RISC-V. The changes have been tested in riscv/riscv-gnu-toolchain by
riscv-dejagnu with riscv-sim.exp/riscv-sim-nano.exp.

libgloss/riscv/crt0.S

@@ -9,6 +9,8 @@
    http://www.opensource.org/licenses.
 */
 
+#include "newlib.h"
+
 #=========================================================================
 # crt0.S : Entry point for RISC-V user programs
 #=========================================================================
@@ -30,9 +32,20 @@ _start:
   sub     a2, a2, a0
   li      a1, 0
   call    memset
+#ifdef _LITE_EXIT
+  # Make reference to atexit weak to avoid unconditionally pulling in
+  # support code.  Refer to comments in __atexit.c for more details.
+  .weak   atexit
+  la      a0, atexit
+  beqz    a0, .Lweak_atexit
+  .weak   __libc_fini_array
+#endif
 
   la      a0, __libc_fini_array   # Register global termination functions
   call    atexit                  #  to be called upon exit
+#ifdef _LITE_EXIT
+.Lweak_atexit:
+#endif
   call    __libc_init_array       # Run global initialization functions
 
   lw      a0, 0(sp)                  # a0 = argc