Add support for illumos cross-compilation

[am11]

Summary of changes:

Essential

• add tryrun configuration for illumos cross-compile.
  • note: src/installer didn't had one before.
• detect ld, and based on that choose linker flags.
  • in cross-compile from Ubuntu, toolchain uses GNU ld, otherwise when we are compiling on SmartOS and other illumos-based distros, it is Solaris ld. Full ld --version output looks like this:

    ld: Software Generation Utilities - Solaris Link Editors: 5.11-1.1763 (illumos)

• link is an llvm concept which does not have a separate equivalent tool in GCC toolchain; gcc -flto provides similar functionality. Perhaps llvm-link is unused in our builds and can be removed?
  • reason why it wasn't caught with GCC builds before:
    1. we always found a totally different "wrong" tool: link(1) via system introspection
    2. in rootfs isolation this unrelated tool is not available, and was failing the build.
• add illumos RID to System.Data.ODBC project.
  • this is the only project which requires RID of new system, otherwise fails the cross-build of src/libraries.
  • rest of the libraries work will follow in separate PR.

Extra

• add armv7-alpine in tryrun scripts.
• prefer ${CMAKE_DL_LIBS} when possible instead of if/else (as already done under src/installer).
• remove trailing slash .tools in .gitignore, so if they are symlinks they don't get added with git add :/

Contributes to: #34944.

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[am11]

cc @janvorli, @jclulow, with this change, the cross-compiled product succeeds dotnet helloworld.dll with coreclr runtime on SmartOS 2020 x64. mono runtime work is done separately: #37560.

[am11]

@janvorli, thanks for the review. I have addressed the feedback. Some blocks were moved under else(CLR_CMAKE_TARGET_WIN32) to avoid the platform check again, therefore easier to follow the diff without a whitespace change: https://github.com/dotnet/runtime/pull/37753/files?w=1.