The only target dependency for most hardware is sizeof(target_phys_addr_t).
Build these files into a convenience library, and use that instead of
building for every target.

Remove and poison various target specific macros to avoid bogus target
dependencies creeping back in.

Big/Little endian is not handled because devices should not know or care
about this to start with.
Signed-off-by: NPaul Brook <paul@codesourcery.com>