unwind: move stacktrace capturing and mmap cache invalidating to trace_syscall_entering

unwind: move stacktrace capturing and mmap cache invalidating to trace_syscall_entering

Instead of handling stacktrace capturing and mmap cache invalidating in
sys_* functions, handle them uniformly in trace_syscall_entering using
new flags introduced by previous two commits.

The patch is simpler than its older version(v3).  The value of
hide_log_until_execve is just ignored.  I found the value is nothing
to do with this patch.  unwind_cache_invalidate is mentioned only
once in trace_syscall_exiting.
Both are suggested by Dmitry Levin.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>