Fix QNX 7.0 support #8593

[dschopf]

These changes resolve all issues described in #8593 except for the linker issue.

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[dschopf]

@googlebot I signed it!

[fowles]

Looks mostly good, but I would like to have Jan take a look because of the aarch64 change in src/google/protobuf/stubs/platform_macros.h

[pablorcum]

@dschopf Please include the missing labels release notes: yes and c++ to enable merging.

[dschopf]

Sorry, I can't do that, looks like I don't have permission to add or edit labels.

[jtattermusch]

I don't have any experience with QNX, but the change in platform_macros.h seems fair
(and it only kicks in when __QNX__ is defined, so I don't think it can break the aarch64 linux support in any way).
I'll let @fowles to review the rest of the code and give the final approval.