QNX: Compiler and feature detection fixes

Tested compiler detection with the QNX 7.0 list of compiler targets:
 * gcc_ntoaarch64le
 * gcc_ntoaarch64le_cxx
 * gcc_ntoarmv7le
 * gcc_ntoarmv7le_cxx
 * gcc_ntox86_cxx
 * gcc_ntoaarch64le_gpp
 * gcc_ntox86_64_gpp
 * gcc_ntoarmv7le_gpp
 * gcc_ntox86_64_cxx
 * gcc_ntox86
 * gcc_ntox86_gpp
 * gcc_ntox86_64

This commit fixes some of the problems described in:
https://cristianadam.eu/20181202/a-better-qnx-cmake-toolchain-file/
Status Job ID Name Coverage
  External
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