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_openat.c

@@ -4,5 +4,5 @@
 /* Open file relative to given directory.  */
 int _openat(int dirfd, const char *name, int flags, int mode)
 {
-  return syscall_errno (SYS_openat, dirfd, name, flags, mode, 0, 0);
+  return syscall_errno (SYS_openat, 4, dirfd, name, flags, mode, 0, 0);
 }