RFC: Fix WITHOUT_SYMVER buildworld

[emaste]

Summary

Move existing standalone -compat source files to build only under MK_SYMVER=yes.

Move __sym_compat macros into #ifdef SYMVER_COMPAT blocks and #define SYMVER_COMPAT in libc's Makefile under MK_SYMVER=yes.

Posted for comment. In general where __sym_compat is included in the main source file I think I'd rather move the compat block into its own file and in the Makefile optionally include it in SRCS, as I did for e.g. fts-compat.c and unvis-compat.c. In some cases it's tricky because the compat code references a static function, and perhaps this -D approach is fine.

https://bugs.freebsd.org/207712

[kib]

It might be less (current and future) work to change the compat macros to not emit versioning directives when SYMVER is not requested. Then we would still compile old code (unconditionally), but not export it from the libc and other basic libraries.

[emaste]

It might be less (current and future) work to change the compat macros to not emit versioning directives when SYMVER is not requested.

At first I thought of having __sym_compat do this directly, but don't want to introduce #ifdefs into the installed system header. (It would have to be opt-out in that case, anyway -- e.g. -DNO_SYM_COMPAT).

I could otherwise do it to the FB10_COMPAT macro in lib/libthr/thread/thr_private.h etc.

Either way I think the Makefile changes make sense where the compat implementation is already provided in its own file.

[emaste]

New approach in D5650