xerces-c add --univeral option

This adds the --universal option to the xerces-c formula.
No change to the xerces-c build process was required
using the standard mechanisms in homebrew for universal builds.
Tested by building and using universal binaries generated
on both 32 bit and 64 bit systems running Snow Leopard.

Closes #26919.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>