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_write.c

@@ -6,5 +6,5 @@
 ssize_t
 _write(int file, const void *ptr, size_t len)
 {
-  return syscall_errno (SYS_write, file, ptr, len, 0, 0, 0);
+  return syscall_errno (SYS_write, 3, file, ptr, len, 0, 0, 0);
 }