libcxx: Add size optimizations

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>

package/libs/libcxx/patches/020-fixes.patch

@@ -0,0 +1,24 @@
+--- a/include/memory
++++ b/include/memory
+@@ -1696,7 +1696,7 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
+             ptrdiff_t _Np = __end1 - __begin1;
+             __end2 -= _Np;
+             if (_Np > 0)
+-                _VSTD::memcpy(__end2, __begin1, _Np * sizeof(_Tp));
++                __end2 = __begin1;
+         }
+ 
+ private:
+--- a/src/filesystem/filesystem_common.h
++++ b/src/filesystem/filesystem_common.h
+@@ -197,8 +197,8 @@ private:
+ using chrono::duration;
+ using chrono::duration_cast;
+ 
+-using TimeSpec = struct ::timespec;
+-using StatT = struct ::stat;
++using TimeSpec = struct timespec;
++using StatT = struct stat;
+ 
+ template <class FileTimeT, class TimeT,
+           bool IsFloat = is_floating_point<typename FileTimeT::rep>::value>