Make frontend endian-aware.

In many parts the DMD frontend assumes a little endian CPU. In some
parts there are checks for endianess but they are incomplete and the
used definition is wrong. (Test for endianess will be removed in dmd
2.062.)
In this commit I add the required #if's and also add a CMake test for
endianess because there is no single compiler definition to check for.
Merge pull request #268 from redstar/endian

Make frontend endian-aware.