osdep: Work around MinGW assert

In several places we use assert(FEATURE), and assume that if FEATURE
is disabled, all following code is removed as unreachable.  Which allows
us to compile-out functions that are only present with FEATURE, and
have a link-time failure if the functions remain used.

MinGW does not mark its internal function _assert() as noreturn, so the
compiler cannot see when code is unreachable, which leads to link errors
for this host that are not present elsewhere.

The current build-time failure concerns 62823083, but I remember
having seen this same error before.  Fix it once and for all for MinGW.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20181022181623.8810-1-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>