HEADS-UP: PIE enabled by default on main

As of 9a227a2fd642 (main-n245052) base system binaries are now built
as position-independent executable (PIE) by default, for 64-bit
architectures. PIE executables are used in conjunction with address
randomization as a mitigation for certain types of security
vulnerabilities.

If you track -CURRENT and normally build WITHOUT_CLEAN you'll need to
do one initial clean build -- either run `make cleanworld` or set
WITH_CLEAN=yes.

No significant user-facing changes are expected from this change, but
there are some minor ones. For example, `file` will indicate that
binaries are PIE by reporting something like `ELF 64-bit LSB pie
executable` rather than `ELF 64-bit LSB executable`. Also, for most
workloads no notable performance impact is expected.

For almost all ports this should result in no change. There are a
small number of ports that use base system /usr/share/mk
infrastructure and thus inherit the base system default, and some of
those initially failed to build. Those found during an exp-run in
PR253275 have been addressed or have patches waiting.

Please watch out for any new issues after you next update the base
system and/or ports, and report issues via a Bugzilla PR or in reply
here.