[ARM32] clang 3.9 as a default for ARM cross build (dotnet/coreclr#11064)

[ARM32] clang 3.9 as a default for ARM cross build (dotnet/coreclr#11064)

* [ARM32] clang 3.9 as a default for ARM cross build

Use clang 3.9 as a default compiler for ARM cross build

* Both arm and armel will be built using clang 3.9 as a default
* ARM CI will use clang 3.9 as a default
* ARM pipieline build will use clang 3.9 as a default

Signed-off-by: Hyung-Kyu Choi <hk0110.choi@samsung.com>
* [ARM32] Update docs describing clang-3.9 and ARM cross build

Signed-off-by: Hyung-Kyu Choi <hk0110.choi@samsung.com>
* [ARM] Check minimum version requirement of clang for ARM cross build

Signed-off-by: Hyung-Kyu Choi <hk0110.choi@samsung.com>
Commit migrated from https://github.com/dotnet/coreclr/commit/0bc55bafbf8b7cde5599fef12c923018de5c5a2b