Use byteswap.h when building against musl libc

[georgthegreat]

Though manpages say bswap_ macros are GNU extensions, both bionic libc and musl libc provide them from the very first releases.

As musl intentionally does not define any specific macro, I suggest changing ifdefs to be based on operating systems rather than on libc's.

[acozzette]

Thanks, @georgthegreat.

[georgthegreat]

Thanks, @acozzette.
As a follow up, I'd like to add a snippet illustrating the problem being fixed.
Whenever a source is dependent on both byteswap.h and protobuf, the following error will occur:

byteswap.h:22:9: error: 'bswap_16' macro redefined [-Werror,-Wmacro-redefined]
#define bswap_16(x) __bswap_16(x)
        ^
google/protobuf/stubs/port.h:321:9: note: previous definition is here
#define bswap_16(x) bswap_16(x)
        ^