Support QNX 7.1 with io-sock+libstd and QNX 8.0 (no_std only)

[flba-eb]

Changes of this pull request:

1. Refactor code for qnx nto targets to share more code in file nto_qnx.rs

2. Add support for an additional network stack on nto qnx 7.1.

   QNX 7.1 supports two network stacks:

   1. io-pkt, which is default
   2. io-sock, which is optional on 7.1 but default in QNX 8.0

   As one can see in the io-sock migration notes, this changes the libc API in a way similar to e.g. linux-gnu vs. linux-musl.

   This change adds a new target which has a different value for target_env, so that e.g. libc can distinguish between both APIs.

3. Add initial support for QNX 8.0, thanks to AkhilTThomas. As it turned out, the problem with forking many processes still exists in QNX 8.0. Because if this, we are now using it for any QNX version (i.e. not check for target_env anymore).

@rustbot label +O-neutrino
CC: @jonathanpallant @japaric @gh-tr @AkhilTThomas