Restore stdint.h for android-3..19 to the original headers

$NDK/tests/standalone/run.sh fail to run with last update on stdint.h
to android-20 level.  Restore stdint.h for fear of the new one breaking
the existing apps

Also fix signal_portable.h to use SIGRTMIN/MAX in old headers
(which got renamed to __SIGRTMIN in new header)

Also change _JBLEN to 1024 which shall be large enough to store CPU
states for forseable future.

Change-Id: Ia9c03558a258c10434eb8e6985adc122532feaa3
3 files changed