[ARM] Change default ABI for AArch32 to be "aapcs" (was "apcs-gnu")

The current default abi when no environment is given is "apcs-gnu",
which is obsolete. This patch changes the default to "aapcs". "aapcs" has both
hard- and soft-float variants, so the -mhard-float, -msoft-float and
-mfloat-abi= options now all behave as expected when no environment is
specified in the triple.

While writing this I also noticed that a preprocessor test claims to be
checking darwin, but is actually checking the defaults, which are
different for darwin.