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_fstat.c

@@ -9,7 +9,7 @@ int
 _fstat(int file, struct stat *st)
 {
   struct kernel_stat kst;
-  int rv = syscall_errno (SYS_fstat, file, &kst, 0, 0, 0, 0);
+  int rv = syscall_errno (SYS_fstat, 2, file, &kst, 0, 0, 0, 0);
   _conv_stat (st, &kst);
   return rv;
 }