MIPS: Fix build error with linuxthreads.old

MIPS: Fix build error with linuxthreads.old

This patch avoid the following build error:
  AS libc/sysdeps/linux/mips/vfork.os
In file included from libc/sysdeps/linux/mips/vfork.S:22:
./libc/sysdeps/linux/mips/sysdep.h:41:20: error: regdef.h: No such file
or directory
libc/sysdeps/linux/mips/vfork.S: Assembler messages:
libc/sysdeps/linux/mips/vfork.S:40: Warning: Unrecognized register name `sp'
libc/sysdeps/linux/mips/vfork.S:40: Warning: Bad .frame directive
libc/sysdeps/linux/mips/vfork.S:40: Error: junk at end of line, first
unrecognized character is `s'
libc/sysdeps/linux/mips/vfork.S:44: Error: illegal operands `dsubu
sp,FRAMESZ'
libc/sysdeps/linux/mips/vfork.S:45: Error: bad or irreducible absolute
expression
libc/sysdeps/linux/mips/vfork.S:65: Error: illegal operands `daddu
sp,FRAMESZ'
libc/sysdeps/linux/mips/vfork.S:69: Error: illegal operands `li a0,0x4112'
libc/sysdeps/linux/mips/vfork.S:70: Error: illegal operands `move a1,sp'
libc/sysdeps/linux/mips/vfork.S:73: Error: illegal operands `li
v0,(5000+55)'
libc/sysdeps/linux/mips/vfork.S:78: Error: illegal operands `bnez
a3,.Lerror'
libc/sysdeps/linux/mips/vfork.S:88: Error: illegal operands `dla
t9,__syscall_error'
libc/sysdeps/linux/mips/vfork.S:90: Error: illegal operands `jr t9'

Signed-off-by: Abdoulaye Walsimou Gaye <awg@embtoolkit.org>
Signed-off-by: Austin Foxley <austinf@cetoncorp.com>