swift234

[Platform] Unify Linux/FreeBSD modulemaps with gyb

A great deal of duplication exists across the Linux and FreeBSD
modulemaps. An Android modulemap would add even more duplication.

Use gyb to conditionally include certain exports for certain
platforms. Most of glibc.modulemap is common across Linux and FreeBSD,
and using gyb we can conditionally include the two exports that differ.