Detect toolchain based on gcc -dumpmachine

Using uname fails e.g. on a 64-bit machine with a 32-bit toolchain.
The following gcc -dumpmachine strings have been verified:
 * 32-bit Linux gives i486-linux-gnu
 * 64-bit Linux gives x86_64-linux-gnu
 * Mac OS X 10.5 gives i686-apple-darwin9
 * MinGW gives mingw32

*darwin8* and *bsd* can safely be assumed to be correct, but *cygwin*
is a guess.

Change-Id: I6bef2ab5e97cbd3410aa66b0c4f84d2231884b05