Shave another uninteresting stack frame off aborts.

With this, stack frame 0 is the abort, not tgkill.

arm:

     #00 pc 0001a41c  /system/lib/libc.so (abort+63)

arm64:

     #00 pc 000000000001d75c  /system/lib64/libc.so (abort+120)

Also "include what you use" for <sys/syscall.h>.

Bug: N/A
Test: ran `crasher abort` and `crasher64 abort`
Change-Id: I6517ac67b39b4133e890d52efc115071c812958b