libgloss: riscv: Fix envp parameter for main

The envp paramter is currently NULL when crt0.S calls main.
With this patch the envp parameter should now be set correctly.
Tested with riscv32 and riscv64: both gdb/sim and qemu do work.

libgloss/riscv/crt0.S

@@ -61,7 +61,9 @@ _start:
 
   lw      a0, 0(sp)                  # a0 = argc
   addi    a1, sp, __SIZEOF_POINTER__ # a1 = argv
-  li      a2, 0                      # a2 = envp = NULL
+  slli    a2, a0, 1 + __SIZEOF_POINTER__ >> 2
+  addi    a2, a2, __SIZEOF_POINTER__
+  add     a2, a2, a1                 # a2 = envp
   call    main
   tail    exit
   .size  _start, .-_start