Support platform QNX OS

[huangweiliang]

Compile with SDP7.1:
./configure --cc=aarch64-unknown-nto-qnx7.1.0-gcc --disable-shm make

Function is verfied by UFS device read and write test.

[axboe]

Thanks for doing this work. Some notes:

1. Please look at commit formatting in the repo in general, most notably you need a proper signed-off-by line in there.

2. All the SA_NOCLDSTOP ifdefs are pretty ugly and also ill formatted. ifdefs go at the start of the line. Please follow coding style. I think this would be much cleaner if you had a generic OS_SA_RESTART, and put a:

#ifndef OS_SA_RESTART
#define OS_SA_RESTART SA_RESTART
#endif

in os/os.h and just have os-qnx.h define it to something else.

3. Ditto the shm/mman changes. Again this mostly looks like it was hacked into existence rather than some thought put into it. Not very maintainable.

[huangweiliang]

I will close this pull request and create new one.
Thanks for your comment.