Restrict default creation of PIE binaries (-fPIE and -pie) to Linux.

Restrict default creation of PIE binaries (-fPIE and -pie) to Linux.
OpenBSD also supports PIE but enables it by default so we don't
need to do anything.  This fixes problems on systems with a version
of GNU ld that accepts -pie but where the run-time linker doesn't
actually support PIE.  Also verify that a trivial PIE binary works
unless PIE is explicitly enabled.