PPC: Resolve references to "this" the same way as normal variables

PPC: Resolve references to "this" the same way as normal variables

Port bd56d279b612f92d2d1d25f92d32fe7f42a455ca

Original commit message:
Make the parser handle references to "this" as unresolved variables, so the
same logic as for the rest of function parameters is used for the receiver.
Minor additions to the code generation handle copying the receiver to the
context, along with the rest of the function parameters.

Based on work by Adrian Perez de Castro <aperez@igalia.com>.

This is a reapplication of https://codereview.chromium.org/1130733003.

R=wingo@igalia.com, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1134073003

Cr-Commit-Position: refs/heads/master@{#28342}