PPC: Make context register implicit for CallInterfaceDescriptors

PPC: Make context register implicit for CallInterfaceDescriptors

Port 7015fd20531ee8498352f82d626c78023a1f6857

Original commit message:
    Up until now the context register was listed explicitly in each stub's
    CallInterfaceDescriptor. This was problematic, because it was listed
    first in the list of register parameters--which is fine for Crankshaft,
    which is more or less built to handle the context as the first
    parameter-- but not ideal for TurboFan, which adds the context at
    the end of all function parameters. Now the context register is no
    longer in the register list and can be handled appropriately by both
    compilers. Specifically, this allows the FunctionType specified for
    each CallInterfaceDescriptor to exactly match the parameter register
    list.

R=danno@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Committed: https://crrev.com/f4eb08f4f48af23324ace5de38ff74615ae1ba67
Cr-Commit-Position: refs/heads/master@{#29421}

[dstence, 2015-07-01 13:38:56]

lgtm

[MTBrandyberry, 2015-07-01 13:43:10]

The CQ bit was checked by mbrandy@us.ibm.com

[commit-bot: I haz the power, 2015-07-01 13:43:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1218463004/1

[commit-bot: I haz the power, 2015-07-01 14:21:08]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-01 14:21:29]

Patchset 1 (id:??) landed as
https://crrev.com/f4eb08f4f48af23324ace5de38ff74615ae1ba67
Cr-Commit-Position: refs/heads/master@{#29421}