Another attempt to work around the problem with old binutils not supporting dmb arguments. Changed 64-bit operations so that the generated asm does not contain offsets in ldrexd and strexd instructions. Otherwise ARM assembler chokes.
Fixed typo in a comment.
Fixed a few bugs with ARM assembler: incorrect comparisons in compare_exchange, compilation failures due to insufficient registers in some optimization modes. Eventually I had to disable 64-bit atomic ops for gcc 4.7 because it is not able to allocate register pairs suitable for ldrexd/strexd instructions.
Adjusted the workaround for ldrexd/strexd instruction problem with register pairs allocation. The problem only affects ARMv6k in Thumb mode, so enable these instructions for ARM mode. Also don't base the decision on the compiler version, it is probably not relevant. Updated lockfree test to reflect the recent changes related to ARM backends.