Replace compile-time dylibs with symlinks to run-time

Replace compile-time dylibs with symlinks to run-time

Dylibs (lib*.so) are installed both into $prefix/lib/rustlib/$triple/lib
for use at compile/link time by rustc, and into $prefix/lib for later
use at runtime by ld.so.  The files are not byte-for-byte identical
since they come from different stages of the compiler build, but they
are expected to be functionally equivalent.

This change simply replaces one set with symlinks to the other, reducing
installed size by about 68MB.