macOS: Do not pass any SDK/-isysroot to compilers by default

On modern macOS, compiler wrappers like `/usr/bin/cc` automatically
choose a SDK to pass via `-isysroot` to an underlying compiler from
Xcode or the CommandLineTools.  Other toolchains like Homebrew's `gcc-*`
come with a default SDK too.  Therefore, when targeting macOS, we no
longer need to choose any SDK or pass an `-isysroot` flag by default.

Update initialization of `CMAKE_OSX_SYSROOT` to be empty by default when
targeting macOS.

Fixes: #19180