* 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

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