This commit helps with platform code separation (fixes#3). It should also help with the build as we can simply include all `impl_*.c` files regardless of OS / arch.
Note: this patch contains breaking changes in `include/cpu_features_macros.h`
- `CPU_FEATURES_OS_LINUX_OR_ANDROID` does not exist anymore
- `CPU_FEATURES_OS_FREEBSD`, `CPU_FEATURES_OS_ANDROID` and `CPU_FEATURES_OS_LINUX` are now mutually exclusive (i.e. `CPU_FEATURES_OS_ANDROID` does not imply `CPU_FEATURES_OS_LINUX`)
- `CPU_FEATURES_OS_DARWIN` has been renamed into `CPU_FEATURES_OS_MACOS` to be able to target non-Mac Apple products (IOS, TV, WATCH). They are now targetable with `CPU_FEATURES_OS_IPHONE`. This matches Apple naming convention described in [this stackoverflow](https://stackoverflow.com/a/49560690).
Visual C++ reported a number of warnings:
..\src\filesystem.c(27): warning C4996: '_open': This function or variable may be unsafe. Consider using _sopen_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.
..\src\filesystem.c(34): warning C4267: 'function': conversion from 'size_t' to 'unsigned int', possible loss of data
..\src\string_view.c(25): warning C4244: 'return': conversion from '__int64' to 'int', possible loss of data
..\src\string_view.c(41): warning C4244: 'return': conversion from '__int64' to 'int', possible loss of data
..\src\utils\list_cpu_features.c(151): warning C4090: 'function': different 'const' qualifiers
..\src\cpuinfo_x86.c(67): warning C4244: 'return': conversion from 'unsigned __int64' to 'uint32_t', possible loss of data
..\test\filesystem_for_testing.cc(48): warning C4267: 'return': conversion from 'size_t' to 'int', possible loss of data
..\test\filesystem_for_testing.cc(57): warning C4267: 'argument': conversion from 'size_t' to 'int', possible loss of data
..\test\bit_utils_test.cc(26): warning C4267: 'argument': conversion from 'size_t' to 'uint32_t', possible loss of data
..\test\bit_utils_test.cc(32): warning C4267: 'argument': conversion from 'size_t' to 'uint32_t', possible loss of data
..\test\bit_utils_test.cc(37): warning C4267: 'argument': conversion from 'size_t' to 'uint32_t', possible loss of data
Most of them, with the exception of _open() -> _sopen_s() are about
questionable implicit casts. Fixing the types and adding explicit casts
to explicitly mark the intention (and silence the warnings in the process)
