Unify build_native via eng/native/build-commons (#1753)

Unify build_native via eng/native/build-commons (#1753)

* Unify build_native via eng/native/build-commons
In order to add new platform, architecture, compiler or its newer
version, there are currently multiple places to update, which can be
deduplicated.

This patch unifies:

* directories creation
* prerequisites checking
* `version.c` generation
* native build invocation
* common argument parsing
* building help menu

for various build scripts under coreclr, installer and libraries.

The common entry-point script is now `eng/native/build-commons.sh` and
rest of the scripts under `eng/native` are implementation detail.

Also extracted CMake platform detection in
`eng/native/configureplatform.cmake`, to share with coreclr and
`installer/corehost`.

* Use if [[ cond ]] in all places