Reduce the likelihood of x64/x86-64 changes breaking the build on other arches (#7083).

Code that needs to differentiate between i386 and x86-64 should now
be written as if x86-64 is the default and i386 is the special case.
Eg:

    # if i386_TARGET_ARCH
    someFuncion = .....
    # else
    someFuncion = .....
    # endif

MERGED from commit 2f7c5785