We had a kRootRegisterBias on x64 before. This CL ports the feature to
all other platforms as well. The root register bias is helpful to adjust
the value of the root register, which allows to better utilize signed
immediate offset constants in load instructions.

We currently use a separate add instruction to add kRootRegisterBias
in the code that initializes the root register. This could be improved
by adding a custom relocation mode ensuring that instead of the root
address, the root address plus the bias is inserted (and in this way
the add instruction can be omitted).

Bug: v8:6666
Change-Id: I55cf02ab85d11e3c6d0d83a8f7905dbf924890f1
Reviewed-on: https://chromium-review.googlesource.com/1113539
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54023}