add visibility=default to exported functions, visibility=hidden to most assembler #2932

Job	Run time
xbuild-x86_64	12m 17s
xbuild-ios	7m 15s
build-windows (make, gfortran, 1, 1)	7m 52s
build-windows (make, gfortran, 0, 0)	5m 52s
build-windows (cmake, gfortran, 1, 0)	26m 33s
build-windows (make, gfortran, 1, 0)	7m 52s
build-windows (cmake, gfortran, 1, 1)	23m 42s
build-windows (cmake, gfortran, 0, 1)	23m 30s
build-windows (cmake, gfortran, 0, 0)	23m 22s
build-windows (make, gfortran, 0, 1)	7m 9s
xbuild-ios32	6m 56s
	2h 32m 20s

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