Port seccomp acceleration to Linux 3.5.0

Port seccomp acceleration to Linux 3.5.0

* etc/NEWS: Update correspondingly.

* exec/Makefile.in (config-mips.m4): Don't define rule
or predicate $(LOADOBJS) on it elsewhere than on MIPS.

* exec/README: Direct developers to GDB.

* exec/trace.c (finish_exec): Resume the tracee after reporting
an error in `exec'.
(after_fork): If seccomp is enabled on Android, and the kernel
is 4.7 or earlier, detect whether revisions to the sequencing of
seccomp events have been backported from 4.8.
(exec_waitpid): Resume the process with PTRACE_CONT after
receiving an unknown signal.
(exec_init): Cease disabling seccomp on Android kernels earlier
than 4.8.

(cherry picked from commit 6b5accdc05d007ab4d1804865c1b043260006673)