lib{c,sys}: fully stub libc verisons of symbols

[brooks]

Summary

When a symbol is to be provided by libsys, create a traping stub in the
dynamic libc rather than linking in a copy of the implementation. This
reduces the size of the dynamic libc and ensures that we're using the
right verison or failing outright should there be further issues with
linkage or the runtime linker.

This change is complicated by the need to for static libc to contain all
the libsys symbols. bsd.lib.mk does provide support for this indirectly
as the SOBJS (shared objects), STATICOBJS (static objects), and
NOSSPSOBJS (position-independent, non-stack-protected, for rtld's use)
variables allow a disjoint set of inputs to be specified for shared and
static libraries, but it's a little awkward.

[brooks]

This whole stack is fixups and cleanups found while making this change to generate the libc.so.7 stubs from the symbols files.

[brooks]

I'm a little hesitant to commit this last change until we're very certain we have the right set of symbols in libsys. Once we do it, we have to be much more cautious about removing symbols from libsys since if something needs them during installworld things will go badly.

[kib]

lib/libsys/Makefile.sys

165

This is lie, of course. But I cannot think about any better way doing it except reverting back to asm and providing some TRAP_INSTR per-arch definition.

BTW, additional advantage of falling to asm is that it could be the same trap instruction for all symbols.