Ensures consistent behaviour and resolves the D-Cache bug (the "exhaustive"
argument being lost due to cache being turned off) when O0 is used.
The changes in this commit are:
- Change -O0 to -Os because "gcc is considered broken at -O0" according to
https://github.com/ARM-software/CMSIS_5/issues/620#issuecomment-550235656
- Use volatile for mem_base so the compiler doesn't optimise away reads or
writes to the SDRAM, which is being tested.
- Use DSB to prevent any other compiler optimisations that would change the
testing logic.
- Use alternating pattern/antipattern in exhaustive test to catch more
hardware/configuration errors.
Implementation adapted by @andrewleech, taken directly from investigation
by @iabdalkader and @dpgeorge.
See #7841 and #7869 for further discussion.
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