Corrected mips code generation for WASM_CALL and JS_TO_WASM_CALL.

The logic for fetching raw call sites needed changing, too, in light
of mips' 2-instruction calls, where using target_address_address is
incorrect. The CL adds platform-specific accessors.

Bug: chromium:793292 chromium:793282
Change-Id: I879ea6bffdad60791d88a6f5ea15087cdcd3f2e9
Reviewed-on: https://chromium-review.googlesource.com/818460Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50037}