want libc functions for extended ucontext_t
This is another part of the 'IPD 38 Signal Handling, Extended FPU State, ucontexts, x86, and You' work. In particular, this provides a set of three new utilities that provide another way to get at and allocate a
ucontext_t so we can include additional state for additional components. This adds a trio of three functions here:
ucontext_alloc()which will allocate a new ucontext_t
ucontext_free()which frees a previously allocated one that was created with this
getcontext_extd()which changes the assumptions around initialization.
For more background, see the IPD.