Add a shim for setenv on PS4 since it does not exist.

A few years back a similar change was made for getenv since neither function is supported on the PS4 platform.

Recently, commit d889d1ef added a call to setenv in compiler-rt which was causing linking errors because the symbol was not found. This fixes that issue by putting in a shim similar to how we previously dealt with the lack of getenv.

Differential Revision: https://reviews.llvm.org/D70033