MIPS: Setup an instruction emulation in VDSO protected page instead of user stack

MIPS: Setup an instruction emulation in VDSO protected page instead of user stack

Historically, during FPU emulation MIPS runs live BD-slot instruction in stack.
This is needed because it was the only way to correctly handle branch
exceptions with unknown COP2 instructions in BD-slot. Now there is
an eXecuteInhibit feature and it is desirable to protect stack from execution
for security reasons.
This patch moves FPU emulation from stack area to VDSO-located page which is set
write-protected for application access. VDSO page itself is now per-thread and
it's addresses and offsets are stored in thread_info.
Small stack of emulation blocks is supported because nested traps are possible
in MIPS32/64 R6 emulation mix with FPU emulation.

Explanation of problem (current state before patch):

If I set eXecute-disabled stack in ELF binary initialisation then GLIBC ignores
it and may change stack protection at library load. If this library has
eXecute-disabled stack then anything is OK, but if this section (PT_GNU_STACK)
is just missed as usual, then GLIBC applies it's own default == eXecute-enabled
stack.
So, ssh_keygen is built explicitly with eXecute-disabled stack. But GLIBC
ignores it and set stack executable. And because of that - anything works,
FPU emulation and hacker tools.
However, if I use all *.SO libraries with eXecute-disabled stack in PT_GNU_STACK
section then GLIBC keeps stack non-executable but things fails at FPU emulation
later.

Here are two issues which are bind together and to solve an incorrect
behaviour of GLIBC (ignoring X ssh-keygen intention) the splitting both issues
is needed. So, I did a kernel emulation protected and out of stack.

Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>