TinyC doesn't support the .symver assembler directive. Add a generic way
to signal this and use that not to define __sym_() macros that use
it. Only use the __sym_* macros in headers when they are defined (which
currently is only for the qsort_r compat code. Not supporting this for
tcc is fine: It's an edge case for legacy binaries / code anyway which
isn't relevant to tinyc.
Sponsored by: Netflix
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45651
