For builtins-mips64.cc, add a static cast to resolve the ambiguity of "0"
around Operand(int64_t) and Operand(const char*).
For mips{,64}/macro-assembler-mips{,64}.cc, remove extra references to masm.

Change-Id: I9ce94c682c64b48501386029a912b0f5e32e268d
Reviewed-on: https://chromium-review.googlesource.com/c/1378365Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58249}