[Patch] Help find "atomicity.h" for target "mingw32"

[Benjamin Kosnik]

>     While trying to bootstrap the latest GCC 3.3 snapshot
>for target "mingw32", I ran into compilation errors about
>"Undefined macro __GTHREAD_MUTEX_INIT". It turns out
>that "atomicity.h" was being picked up from "cpu/generic"
>rather than "cpu/i486" ("mingw32" is an alias for
>"i386-pc-mingw32", so target_cpu is set to "i386").

Your patch will work for all cases where mingw32 is actually i486 and
above, but I think a better approach might be to fix up the aliases so
that target_cpu is accurate. This would mean

for i386 and below
"i386-pc-mingw32" -> target_cpu == i386 

for i486 and above
"i386-pc-mingw32" -> target_cpu == i[456]86 

-benjamin