BUILD: ist: use the literal declaration for ist_lc/ist_uc under TCC

BUILD: ist: use the literal declaration for ist_lc/ist_uc under TCC

TCC doesn't knoow about __attribute__((weak)), it silently ignores it.
We could add a "static" modifier there in this case but we already have
an alternate portable mode that is based on a slightly larger literal
for obsolete linkers (and non-ELF systems) which choke on weak. Let's
just add the test for tcc there and use it in this case.

No backport is needed.