Only pass the minimum number of syscall arguments

Previously, __internal_syscall() compiled into asm-code that unconditionally
sets the syscall argument registers a0 to a5.

For example, the instruction sequence for a exit syscall looked like
this:

    li    a0, 1   # in ther caller of exit()
    # ...         # in newlib:
    li    a1, 0   # unused arguments
    li    a2, 0
    li    a3, 0
    li    a4, 0
    li    a5, 0
    li    a7, 93  # exit syscall number

(i.e. the binary contains then 5 superfluous instructions for this
one argument syscall)

This commit changes the RISC-V syscall code such that only the required
syscall argument registers are set.

GCC detects that argc is known at compile time and thus evaluates all the
if-statements where argc is used at compile time (tested with -O2 and -Os).

libgloss/riscv/sys_sbrk.c

@@ -41,13 +41,13 @@ _sbrk(ptrdiff_t incr)
 
   if (heap_end == 0)
     {
-      long brk = __internal_syscall (SYS_brk, 0, 0, 0, 0, 0, 0);
+      long brk = __internal_syscall (SYS_brk, 1, 0, 0, 0, 0, 0, 0);
       if (brk == -1)
         return (void *)__syscall_error (-ENOMEM);
       heap_end = brk;
     }
 
-  if (__internal_syscall (SYS_brk, heap_end + incr, 0, 0, 0, 0, 0) != heap_end + incr)
+  if (__internal_syscall (SYS_brk, 1, heap_end + incr, 0, 0, 0, 0, 0) != heap_end + incr)
     return (void *)__syscall_error (-ENOMEM);
 
   heap_end += incr;