compile failure on ARMv7 hosts when compiled for thumb if --enable-debug

[Peter Maydell]

QEMU won't compile if you configure --enable-debug and gcc is building in Thumb2.

This is because we have picked r7 as the TCG_AREG0 (fixed register for holding the CPU environment pointer), which clashes with its use as the frame pointer in Thumb. (ARM compilation is fine because the frame pointer is a different register there.)

We could fix this by forcing -fomit-frame-pointer when compiling the relevant source files (which is what we do on x86 where register pressure forces us to use EBP for AREG0) but it would be much better to just move AREG0 to something else. We can use r6 if we want to stick with a low-reg, or move up to r8 if we think a high-reg likely to be a better choice.

TODO:
 * compile with both r6 and r8, and do a trivial benchmarking run
 * submit patch (before the upstream 1.0 freeze!)

NB: AREG0 is set in two places that need to stay in sync: dyngen-exec.h and tcg/arm/tcg-target.h

[Peter Maydell (pmaydell)]

Patch for this: http://patchwork.ozlabs.org/patch/133202/