Carl Mehner reported that Mac OS X's ar(1) does not support the D flag
that tells it build a deterministic archive.

According to Debian[1], recent versions of GNU |ar| will build
determistic archives by default anyway, so there's not a pressing need
to force the issue.

[1] https://wiki.debian.org/ReproducibleBuilds/TimestampsInStaticLibraries

Carl Mehner reported that the build fails on Mac OS X using its
normal Apple Clang toolchain because that toolchain requires a slightly
different set of perlasm and linker options.

This commit introduces the |TARGET_OS| build option to allow us to make
operating-system-dependent choices during the build. It isn't ideal to
use |TARGET_OS| for this problem because it's really an issue with the
toolchain (a host issue), but this seems to be a reasonable first step
while we figure out a long-term solution.

Make the Makefile require a standard
|TARGET=<arch>-<vendor>-<sys>-<abi>| variable to be set instead of
requiring |TARGET_OS| and |TARGET_ARCH_BASE|.

Also, have the Makefile generate outputs to a subdirectory of build/
that names the target and compiler. This will make it easier and less
error-prone to test multiple configurations using a single source tree,
and will make some future cross-compilation targets easier.