Add netsurf static contant to the base system.
Add cheri_system_calloc and cheri_system_free to allocate memory via the
cheri_system object.  This allows objects to be allocated in the sandbox
that survive a reset of the sandbox.

This implementation may leak memory on sandbox failure or reset.
Trivial test for cheri_system_calloc() and cheri_system_free().
Actually call clock_gettime method.