tcg/optimize: Add fallthrough annotations

tcg/optimize: Add fallthrough annotations

To be able to compile this file with -Werror=implicit-fallthrough,
we need to add some fallthrough annotations to the case statements
that might fall through. Unfortunately, the typical "/* fallthrough */"
comments do not work here as expected since some case labels are
wrapped in macros and the compiler fails to match the comments in
this case. But using __attribute__((fallthrough)) seems to work fine,
so let's use that instead (by introducing a new QEMU_FALLTHROUGH
macro in our compiler.h header file).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201211152426.350966-11-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>