Changed standard to 2a. 2a (as well as 17) contain more constexpr
functions, which are evaluated at compile time. This saves space.
Added --gc-sections. With the CXXABI change, this now makes the package
smaller.
With these, size went down to 210845 on mipsel_24kc.
Also fixed two small compiler warnings. No real change in behavior.
Signed-off-by: Rosen Penev <rosenp@gmail.com>
