Consolidate objcopy detection

[am11]

strip_destination_file file is defined in :/eng/native/functions.cmake, which is already included by top-level cmake scripts in coreclr, installer and libraries. This PR de-duplicates that logic and simplifies condition blocks in eng/native/configuretool.cmake (where unified tool detection takes place).

[am11]

@janvorli, @jkotas, can you please take a look?
Failure is about SDK not found

curl: (22) The requested URL returned error: 404 Not Found
dotnet-install: Cannot download: https://dotnetcli.azureedge.net/dotnet/Sdk/5.0.100-preview.2.20155.14/dotnet-dev-ubuntu.16.04-x64.5.0.100-preview.2.20155.14.tar.gz
dotnet_install: Error: Could not find/download: `.NET Core SDK` with version = 5.0.100-preview.2.20155.14
dotnet_install: Error: Refer to: https://aka.ms/dotnet-os-lifecycle for information on .NET Core support
##[error](NETCORE_ENGINEERING_TELEMETRY=InitializeToolset) Failed to install dotnet SDK from public location (exit code '1').
/root/runtime/eng/common/tools.sh: line 418: /root/runtime/.dotnet/dotnet: No such file or directory
##[error](NETCORE_ENGINEERING_TELEMETRY=Build) Build failed (exit code '1').

[am11]

If we try to invoke libraries build on unsupported platform, we get error from line 27 in this file: clr_unknown_arch macro undefined, instead of Only AMD64, ARM64, ARM and I386 are supported message from that function (defined in functions.cmake). Therefore, this inclusion here makes sense (as opposed to including functions.cmake before configureplatform.cmake at all callsites).

[janvorli]

LGTM, thank you! I didn't even know that we have the strip_symbols at so many places.

[am11]

Thanks. There is still install_symbols and install_library_and_symbols functions in installer and libraries respectively, which could be moved to functions.cmake. The libraries has hard-coded destination PDB which makes it a bit different, so I did not touch it as part of this PR.