Delay get_regs invocation

Delay get_regs invocation

strace used to call get_regs on every stop of a tracee, including cases
when the result is going to be discarded.  Prepare to change this lavish
practice by delaying get_regs invocation until its result is actually
needed.

* defs.h (get_regs): Remove.
* strace.c (trace): Do not call get_regs.
* syscall.c (get_regs): Add static qualifier.
(trace_syscall_exiting, print_pc, get_scno): Call get_regs.