You are doing it wrong. ^__~
You should not modify the system environment, simply use the export command in the shell, without the need to reboot the system:
"export PS4SDK=~/path/to/ps4-payload-sdk" or if inside the SDK dir "export PS4SDK=$(pwd)" less typing.
If GCC is already available, it can be used instead of installing CLANG.
And it seems the version of binutils to use is critical to avoid errors at linking time, the provided linker script seems to target binutils 2.25 or earlier, with 2.28 (the version available on Debian Stretch) the ld command