* complete abs, fmod and sin function in simplemath.h

* remove some unused variables in simplemath.cpp

* modify test-coverage.yml and add some functions to simplemath.cpp

* modify erf.cpp which included math.h

* include platform.h for NCNN_SIMPLEMATH definition

* move utility constants and functions in simplemath.h to simplemath.cpp

* guard simplemath functions with extern "C"

* add NCNN_EXPORT macro in simplemath.h

* include plateform.h and guard all declarations with NCNN_SIMPLEMATH

* clean unused code in test_unaryop.cpp

* guard #include <vector> with NCNN_SIMPLEMATH in benchncnn.cpp

* add 'static' to guard functions that not declarated in header file

* modify sin and cos with better implementation

---------

Co-authored-by: HonestDeng <HonestDeng@users.noreply.github.com>

* [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

* Fix warning C4244 in src/layer/normalize.cpp

C4244: '=': conversion from 'double' to 'float', possible loss of data

* Fix warning C4244 in src/layer/requantize.cpp

C4244: 'initializing': conversion from 'double' to 'int', possible loss of data

* Fix warning C4244 in src/mat_pixel_resize.cpp

C4244: '=': conversion from 'double' to 'int', possible loss of data

* Fix warning C4244 in src/mat_pixel.cpp

C4244: '=': conversion from 'int' to 'float', possible loss of data

* Fix warning C4267 in src/modelbin.cpp

C4267: 'initializing': conversion from 'size_t' to 'int', possible loss of data

* Fix warning C4244 in src/layer/batchnorm.cpp

C4244: 'initializing': conversion from 'double' to 'float',
    possible loss of data

* Fix warning C4244 in src/layer/padding.cpp

C4244: 'argument': conversion from 'const float' to 'T', possible loss of data

* Fix warning C4244 in src/layer/priorbox.cpp

C4244: '=': conversion from 'double' to 'float',
    possible loss of data
C4244: 'initializing': conversion from 'double' to 'float',
    possible loss of data