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

Review URL: https://codereview.chromium.org/1230473006

Cr-Commit-Position: refs/heads/master@{#29548}