pmc: Fix some problems with the makefile

• For some reason we don't build it as a PIE, but I don't have any problems doing so with either clang or gcc.
• There is no apparent need to override WARNS, so don't.
• Some building with -O0, presumably that's left over from debugging.

MFC after: 1 week
Reviewed by: imp, brooks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D43923