[FR] support any platform through user-provided functions

**Is your feature request related to a problem? Please describe.**
I'd like to use the benchmark suite in a restricted environment which differs from the currently supported platforms, like linux or windows. There might not be a steady clock provided through the `libstdc++`, nor are `pthreads` available but a compatibility layer and ultimately my platform exposes its own functions to measure CPU usage for example.

**Describe the solution you'd like**
I've seen there are already [platform-specific switches](https://github.com/google/benchmark/blob/main/src/timers.cc#L142-L179) in-place in some functions. I'd like to be able to provide my own implementations or fallbacks for some parts of the suite.

I could think of the following categories, where that would be useful, also including any errors I got:
* **output streams/console**: The user could provide their own `std::cout`, console [color detection](https://github.com/google/benchmark/blob/main/src/colorprint.cc#L184), etc.
* **thread library**: Since this is [detected by CMake](https://github.com/google/benchmark/blob/main/CMakeLists.txt#L295), there's not much you could to at compile-time.
* **timing functions**: Provide your own `ProcessCPUUsage`, `ThreadCPUUsage`, etc.
* [**cycle clock**](https://github.com/google/benchmark/blob/main/src/cycleclock.h#L219)
* **system info**: Like [`GetSystemName`](https://github.com/google/benchmark/blob/main/src/sysinfo.cc#L447)

**Describe alternatives you've considered**
I couldn't really think of any, yet. Maybe I need to write a linux compatible layer, so that the suite would use the linux API, which internally translate all calls according to my platform, but that is quite the undergoing.

**Additional context**
I acknowledge that my request is an bedge-case. However, I think, that by making it possible to provide user-defined fallbacks for all sorts of functionality of the suite, makes it more attractive for other platform and improves its compatibility.