feat: syscall hooks

[ndrewh]

Dynamorio supports syscall hooks:

• pre-syscall dr_register_pre_syscall_event()
• post-syscall dr_register_post_syscall_event()
• and the ability to filter the above events... dr_register_filter_syscall_event().

I suspect we should implement the pre- and post- hooks in pyda_core, and filter syscall events in the Python wrapper. If the overhead is too substantial, then we can implement a simple version of the filter that just checks the syscall number against a list of registered events.

Proposed API:

# Syscall hooks use the same signature as regular hooks
p.pre_syscall(syscall_num, hook)
p.post_syscall(syscall_num, hook)

This should support multiple hooks for a single syscall, but that support can be provided by the Python wrapper.