rts: Fix Arena blocks accounting for MBlock sized allocations
When requesting more than BLOCKS_PER_MBLOCK blocks
allocGroup can return a
different number of blocks than requested. Here we use the number of
requested blocks, however
arenaFree will subtract the actual number of
blocks we got from arena_blocks (possibly) resulting in a negative value
ASSERT(arena_blocks >= 0).
- are either individually buildable or squashed
- have commit messages which describe what they do
- add a testcase to the testsuite.