-pie triggers a bug in glibc's linker when used with goma
(https://sourceware.org/bugzilla/show_bug.cgi?id=16381). Allow
the clang build to disable -pie for host modules through the
DISABLE_HOST_PIE environment variable so it can produce a toolchain
that works with goma.
Bug: 15814177
Bug: 34722791
Change-Id: Ic664a1b821aaeaf2bde14b0afa1a1975e31300cb