Fix a series of compiler warnings

Clang found some issues with the Yasm code base. Previously we ignored
all warnings, including one where clang found that we had invalid memory
access.

We now fix some warnings, and make the warnings we suppress explicit.
Hopefully this will fix some of the build flakiness we see on MSVC where
we rely heavily on yasm.

Bug: b/147320144
Change-Id: If3def59bd1c8c4b4f78fd72706869f3e93fd84cf
6 files changed