x86-win32 registry lookup patch

[Richard Kenner]

    The current code in prefix.c looks up GCC installation paths in the
    windows registry; If the keys exist in the registry howerver, it has
    the effect of screwing up other possibly incompatible versions of GCC
    on the same system by essentially defining a nicely hidden value for
    GCC_EXEC_PREFIX.

The point is that you are now supposed to use GCC_ROOT.  The reason
for this method over the old one is that you can now relocate both
native and cross-compilers.  I think disabling this option is a very
bad idea.  We should instead encourage its use.

Moreover, the above isn't quite correct: all this does is change the *default*
prefix used: GCC_EXEC_PREFIX continues to override that part of the default.