Split target/Config.in.arch into multiple Config.in.* in arch/

Split target/Config.in.arch into multiple Config.in.* in arch/

target/Config.in.arch had become too long, and we want to remove the
target/ directory. So let's move it to arch/ and split it this way:

 * An initial Config.in that lists the top-level architecture, and
   sources the arch-specific Config.in.<arch> files, as well as
   Config.in.common (see below)

 * One Config.in.<arch> per architecture, listing the CPU families,
   ABI choices, etc.

 * One Config.in.common that defines the gcc mtune, march, mcpu values
   and other hidden options.

[Peter: space->tab fix, mipsel64 little endian, mips3 as noted by Arnout]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>