add a new generic x86 machdep directory

this merges i386 and x86_64 into a single dir.
the few differences use #ifdef __x86_64__.
make md-amd64-gcc & md-i386-gcc symlinks to x86
configure: __x86_64__ is set automatically by the compiler

if we let configure add this when it (erroneously) detects amd64
(due to a 32bit-crosscompiler build on a 64 bit kernel), we break
the 32bit build.
qemuuaeglue.h: fix problem when compiled with gcc 4.5.4
Merge pull request #38 from rofl0r/linux_i386

Linux i386