Fix #2120, Support clang for whole archive flags

[skliper]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to [email protected] and copied [email protected].

Describe the contribution

• Fix Account for Clang based toolchains in CMake logic for --whole-archive flag generation #2120
• Adds a toolchain file for clang to make testing simple

Testing performed
W/ Clang 10 installed, performed the following changes in sample_defs:

• Removed unsupported flags
• Compiled permissive to confirm it worked w/ standard user privilages
• Explicitly set cpu1 to use the clang toolchain

diff sample_defs/arch_build_custom.cmake cfe/cmake/sample_defs/arch_build_custom.cmake
36a37,38
>     -Wno-format-truncation      # Inhibit printf-style format truncation warnings
>     -Wno-stringop-truncation    # Inhibit string operation truncation warnings
diff sample_defs/default_osconfig.cmake cfe/cmake/sample_defs/default_osconfig.cmake
36d35
< set(OSAL_CONFIG_DEBUG_PERMISSIVE_MODE TRUE)
diff sample_defs/targets.cmake cfe/cmake/sample_defs/targets.cmake
107d106
< SET(cpu1_SYSTEM i686-linux-clang)

Expected behavior changes
Properly sets the whole archive flags for clang, warns if compiler is unmatched

System(s) tested on
CI shows no impact to base system
Steps above to prove Clang works

Additional context
Could add to CI if someone wanted to maintain support, might be overcome if there's a target added to CI that uses clang.
Clang build does depend on nasa/sample_lib#92 being merged, but doesn't impact current CI

Third party code
None

Contributor Info - All information REQUIRED for consideration of pull request
Jacob Hageman - NASA/GSFC