Add PT_GNU_PROPERTY enum (#319)

Add PT_GNU_PROPERTY enum (#319)

This commit adds the missing `PT_GNU_PROPERTY` program header enums.

Additional information regarding the `PT_GNU_PROPERTY` can be found at:
* https://reviews.llvm.org/D70959
* https://github.com/hjl-tools/linux-abi/wiki/linux-abi-draft.pdf (linked in above url)
* https://sourceware.org/pipermail/libc-alpha/2020-May/113841.html (commit in libc adding this value)

This program header can be found, e.g., in a glibc in Ubuntu 20.04 (see `docker run --rm -it ubuntu:20.04 cat /usr/lib/x86_64-linux-gnu/libc-2.31.so > libc-2.31.so`).