ARMv6 has the same basic barriers as ARMv7+, but they are accessed using
the CP15 coprocessor. This patch allows the assembler to select the
appropriate instruction.

This also fixes TurboFan's atomic loads and stores for ARMv6 platforms.

BUG=

Review-Url: https://codereview.chromium.org/2318553002
Cr-Commit-Position: refs/heads/master@{#39203}