Switch x86/x64 trampolines to use __attribute__((naked))

Switching to clang++ means __attribute__((optimize)) is no longer
recognized. However, we do get __attribute__((naked)).

Slightly larger change than I'd like because __attribute__((naked))
doesn't permit C variable declarations.

Change-Id: I558ccaf292e381d33e10c5034053cdc651eff5f2
Bug: b/79574249
Test: TBC