microblaze port committed

[Rich Felker <dalias@aerifal.cx>]

Hi all,

I've committed the initial (and seemingly fully-working) version of
the microblaze port. Several caveats:

1. Upstream binutils has a serious bug in gas whereby the relocations
   it generates for symbols with local weak definitions cannot be
   resolved by the linker. Patch at:
   http://permalink.gmane.org/gmane.linux.uclinux.microblaze/11155

2. The toolchain binaries from Xilinx seem to produce .o files
   incompatible with standard binutils, and vice versa. Pick one or
   the other; don't mix them.

3. Threads are untested because qemu is broken, at least my version of
   qemu (app-level emu) has a bug where clone() does not advance the
   program counter properly in the child, so it starts a forkbomb-like
   cascade of clone syscalls (thankfully just linear growth rather
   than exponential).

4. Until very recently, qemu did not support the lwx/swx instructions
   needed for implementing atomics, and treated them instead as plain
   lw/sw (load/store). I've added one otherwise-wasteful instruction
   to the atomic asm in atomic.h so that atomic cas/swap will at least
   behave as an equivalent non-atomic operation on older qemu, rather
   than randomly going into infinite loop. Older hardware may be
   affected by this issue too.

I'm sure we'll find other problems once there's a better build/test
environent in-place, but overall this seems to be the best
initial-commit quality of any port yet, probably due to the fact that
the microblaze ISA+ABI is the sanest, simplest ISA+ABI I've ever coded
to.

Rich