Don't reserve R2 on Darwin/PPC

Now that only the register-scavenger version of the CR spilling code remains,
we no longer need the Darwin R2 hack. Darwin can use R0 as a spare register in
any case where the System V ABI uses it (R0 is special architecturally, and so
is reserved under all common ABIs).

A few test cases needed to be updated to reflect the register-allocation changes.
Correctly compute linkage of decls forward declared extern C.

This fixes a crash in

namespace {
  struct X {};
}
extern "C" X test2_b;
X test2_b

before we would assign different linkages to each of the test2_b decls.