libc: fix MIPS N64 fork

libc: fix MIPS N64 fork

fork() is broken for MIPS64 N64 ABI. You can check it with a simple
C program statically linked with qemu-mips64 user emulation.
Internally fork() is using the clone system call (at least with NPTL)
with 5 arguments. See ./libpthread/nptl/sysdeps/unix/sysv/linux/i386/fork.c.
The calling conventions for MIPS N32 and N64 allow to use up to 8 registers
for that. See http://en.wikipedia.org/wiki/Calling_convention#MIPS
This is correctly implemented in libc/sysdeps/linux/mips/bits/syscalls.h,
but not in libc/sysdeps/linux/mips/sysdep.h. fork.c uses the later one.
It seems that fork() works fine for MIPS64 N32 with just using the stack like
with the O32 case. There is a user of INLINE_SYSCALL with 7 arguments in
libc/sysdeps/linux/common/sync_file_range.c for MIPS64 N32, so I decided to
only use the macros for the MIPS64 N64 case. With this patch my uClibc based
Linux system boots up fine in qemu-system-mips64.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>