Upstream support for a single WORLDTMP for libcompat builds

[bsdjhb]

Summary added by @brooksdavis:

On FreeBSD when doing builds for freebsd32 support, a separate temporary sysroot is created. It contains the compat architecture's headers and installed libraries as they are built. It is referred to as LIBCOMPATTMP. On CheriBSD we merged LIBCOMPATTMP into WORLDTMP (the default sysroot). This worked for MIPS (were 32 and 64-bit code uses the same headers) and currently works for aarch64 and riscv64 because they don't have freebsd32 support. In theory it should work for amd64 if -m32 compilation works, but it doesn't quite as (at least) sys/cpufunc.h is not compatible and thus csu bits don't build. At this point it's unclear how many layers the onion has...

[brooksdavis]

https://reviews.freebsd.org/D34697

[jrtc27]

Upstreamed