x86: enable recoverable exceptions on 64-bit

These were previously assumed to always be fatal.
We can't have the faulting thread's XMM registers
clobbered, so put the SIMD/FPU state onto the stack
as well. This is fairly large (512 bytes) and the
execption stack is already uncomfortably small, so
increase to 2K.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>