libc/quad: narrow list of symbols exposed on i386

These symbols aren't present on i386 so don't try to expose them.

Given the structure of quad/Makefile.inc, it might make more sense to
have per-arch symbol maps here, but this is sufficent to build with
WITHOUT_UNDEFINED_VERSION on i386.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44243