Fix two register leaks in the trusted->untrusted context switch

Fix two register leaks in the trusted->untrusted context switch

x86-32: Stop leaking the address of the NaClThreadContext in %ecx.

ARM: Stop leaking the value of r12 from trusted code.

Add a comprehensive test to check that registers are reset when a
syscall returns.

Add some comments about resetting flags.  On x86-64, reorder an
instruction which resets flags so that the flags don't depend on the
return address, to ease testing.

Move UnsetNonCalleeSavedRegisters() out of suspend_test_guest.c into
tests/common.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2919
TEST=run_syscall_return_regs_test

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=9343

[bsy, 2012-08-01 17:24:46]

comment nit.  plz fix.  o/w lgtm.

btw, might also note aliase r12==ip

http://codereview.chromium.org/10829110/diff/1/src/trusted/service_runtime/ar...
File src/trusted/service_runtime/arch/arm/nacl_switch.S (right):

http://codereview.chromium.org/10829110/diff/1/src/trusted/service_runtime/ar...
src/trusted/service_runtime/arch/arm/nacl_switch.S:25: * We clear registers r2,
r3, lr, flag and status fields in CPSR
and r12

[Mark Seaborn, 2012-08-01 17:46:06]

http://codereview.chromium.org/10829110/diff/1/src/trusted/service_runtime/ar...
File src/trusted/service_runtime/arch/arm/nacl_switch.S (right):

http://codereview.chromium.org/10829110/diff/1/src/trusted/service_runtime/ar...
src/trusted/service_runtime/arch/arm/nacl_switch.S:25: * We clear registers r2,
r3, lr, flag and status fields in CPSR
On 2012/08/01 17:24:46, bsy wrote:
> and r12

Fixed, and mentioned its alias, 'ip'.  Thanks.