Add cross build support (relying in compiler's -dumpmachine).
Misc tweaks to not break native builds and improve cross builds.

- When running build.sh make sure we don't use CFLAGS from env (might have cross cflags).
- When testing for compiler features, test the compiler's target not native.
- If HOST_CC is clang, or CC is gcc or not set, use native tools in toolchain wrappers.
Bunch of minor edits and sanity checks to crossbuild support.
Change from cc -dumpmachine to cc -v.

cc -dumpmachine isn't supported by every compiler (e.g. clang 1.1).
Also, update arch -> NetBSD machine mapping for cc target instead
of what uname reports.
add missing semicolon demanded by solaris sed
fix -32 support detection
fix variable subst
need a bit more than $mach_arch for toolchain