[phobos] 64 & 32 bit libraries for OSX

[Walter Bright]

On Linux, the 64 bit and 32 bit libraries exist along separate paths, and we put 
both paths on the command to the linker:

DFLAGS=-I%@P%/../../src/phobos -I%@P%/../../src/druntime/import 
-L-L%@P%/../lib32 -L-L%@P%/../lib64
-L--no-warn-search-mismatch -L--export-dynamic

However, attempting to do this on OSX fails (using -L-no_arch_warnings instead 
of -L--no-warn-search-mismatch). It apparently always tries to link in the first 
library found, not the first one with matching architecture.

What is the right way on OSX to do a dual-architecture library?