Revert of [arm] Decrease the size of the assembler class by allocating buffers of pending constants on the he… (patchset #2 id:20001 of https://codereview.chromium.org/1309903009/ )

Reason for revert:
Static assert failed on ARM64

Original issue's description:
> [arm] Decrease the size of the assembler class by allocating buffers of pending constants on the heap.
>
> BUG=chromium:521828
> LOG=N
>
> Committed: https://crrev.com/033af3fa511c52bc4049cd278d0623a6c6f9f9c3
> Cr-Commit-Position: refs/heads/master@{#30592}

TBR=jkummerow@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:521828

Review URL: https://codereview.chromium.org/1307863007

Cr-Commit-Position: refs/heads/master@{#30593}