There was a syntax error introduced in bug 1796380 that removed the flag
entirely. In practice, it didn't make a difference for us because clang
now defaults to PIE, but it can make a difference in environments where
the compiler is not using such a default.
However, on both Darwin and WASI, clang complains the flag is unused, so
we have to do something about it.
Differential Revision: https://phabricator.services.mozilla.com/D180279
ca2f2016f6