xtensa: make it work with TLB misses during interrupt handling

If there are any TLB misses during interrupt handling,
the user, kernel and double exception vector will be triggered
for the miss and the DEPC and EXCCAUSE overwritten as the TLB
missse are be handled in the assembly code and execution
returned to the original vector code. Because of this, both
DEPC and EXCCAUSE being read in the C handler are not the ones
that triggered the original exception (for example, level-1
interrupt). So stash both DEPC and EXCCAUSE such that
the original cause of exception is visible in the C handler.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>