mips: FIx clone3 implementation (BZ 31325)

For o32 we need to setup a minimal stack frame to allow cprestore
on __thread_start_clone3 (which instruct the linker to save the
gp for PIC).  Also, there is no guarantee by kABI that $8 will be
preserved after syscall execution, so we need to save it on the
provided stack.

Checked on mipsel-linux-gnu.

Reported-by: Khem Raj <raj.khem@gmail.com>
Tested-by: Khem Raj <raj.khem@gmail.com>