[turbofan] Use NumAllocatableRegisters() instead of kMaxNumAllocatableRegisters.

[turbofan] Use NumAllocatableRegisters() instead of kMaxNumAllocatableRegisters.

Use NumAllocatableRegisters() instead of kMaxNumAllocatableRegisters in
ArchDefaultRegisterConfiguration::ArchDefaultRegisterConfiguration()
otherwise the compiler DCHECKS if triggered during snapshot builds on
Arm where only 16 DoubleRegisters are avaiable instead of 32.

BUG=v8:4280
LOG=N

Committed: https://crrev.com/a4584c07e77bc0f5bf7fc2ee73a2194954fa64d4
Cr-Commit-Position: refs/heads/master@{#29548}

[rmcilroy, 2015-07-08 15:25:47]

The CQ bit was checked by rmcilroy@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-08 15:26:02]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1230473006/1

[rmcilroy, 2015-07-08 15:27:11]

rmcilroy@chromium.org changed reviewers:
+ bmeurer@chromium.org

[rmcilroy, 2015-07-08 15:27:12]

This cropped up when I was generating the interpreter prototype in the snapshot.
I'm not sure why it didn't happen for the current TurboFanCodeStubs.

Benedikt, PTAL, thanks.

[commit-bot: I haz the power, 2015-07-08 15:48:28]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-08 15:48:29]

Dry run: This issue passed the CQ dry run.

[Benedikt Meurer, 2015-07-08 19:11:10]

lgtm

[rmcilroy, 2015-07-09 09:12:29]

The CQ bit was checked by rmcilroy@chromium.org

[commit-bot: I haz the power, 2015-07-09 09:12:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1230473006/1

[commit-bot: I haz the power, 2015-07-09 09:14:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-09 09:14:48]

Patchset 1 (id:??) landed as
https://crrev.com/a4584c07e77bc0f5bf7fc2ee73a2194954fa64d4
Cr-Commit-Position: refs/heads/master@{#29548}