* [build] add toolchain file w/o stdcxx dependency

* [build] link m and gcc lib explicitly

* [ncnn] complete simple stl impl

* [ncnn] adapt for ncnn simplestl

* [test] adapt for ncnn simplestl

* [ncnn] fix missing algorithm and list when simplestl disabled

* [ncnn] fix guard for operator new and delete

* [style] fix the code style

* [build] fix build failed on darwin and emscripten

* [ci] do not import cxx to avoid operator conflict

* [ncnn] add temporary partial_sort impl using bubble sort

heap sort should be used for better perf.

* [ncnn] add std greater and less function

* [ncnn] fix placement new operator overload

* [ncnn] add operator delete with size info

* [build] disable exception, rtti, example and tools when simplestl on

* [build] add toolchain for arm simplestl

* [build] add toolchain for aarch64 simplestl

* [ncnn] move initializer to constructor

* [ncnn] use deteiled type instead of auto

* [ncnn] use plain lib name in target_link_libraries

* Implementation for hard sigmoid

Not yet tested.

* Resolve requests