ucalls: Trace syscalls

Add `-S` switch for tracing syscalls with or without high-
level language method calls. When `-S` is provided, attach
to all `sys_` and `SyS_` entry points in the kernel and
trace the number of calls and execution latency (if `-L`
was also specified).

This is now also a useful general-purpose tool for syscall
count/latency summarizing when a language is not specified.