[compiler-rt][NFC] add note to interface_symbols_linux.cpp about darwin

There were changes made to the linux version of this test that were not made for darwin
(see https://reviews.llvm.org/D115837) and this caused downstream failures.

Adding comment to this test to remind people to edit interface_symbols_darwin.cpp.
There is the reverse of this comment in the darwin file to remind us to edit the linux version already.

Differential Revision: https://reviews.llvm.org/D115899