Fix some clang compilation issues.

Use expected inline behavior with clang.

GCC defaults to -std=gnu90, giving C89 inline semantics with GNU
extensions. Clang defaults to C99. Explicitly use gnu90.

Mark an unused parameter as __unused.

Fix some incorrect casts.

Change-Id: I05b95585d5e3688eda71769b63b6b8a9237bcaf4