Correct definition of 'struct sigcontext' on MIPS.

Add missing fields ('sr' and 'mc_tls') to 'struct sigcontext'.

The kernel doesn't use 'struct sigcontext' but instead uses 'ucontext_t'
which includes 'mcontext_t' in 'struct sigframe' to build the signal frame.
As a result, this change is not an ABI change but simply making
'struct sigcontext' correct. Note that 'struct sigcontext' is only used
for "Traditional BSD style" signal handlers.

While here, rename the 'xxx' field to 'spare' to match 'mcontext_t'.

Sponsored by: DARPA, AFRL