CMakeDetermineCompilerABI: Use normal linker flags in ABI project

When compiling the ABI detection test project, do not override
CMAKE_EXE_LINKER_FLAGS completely.  The normally selected value of this
variable may influence how the link is done and may be needed to be
representative of how the calling project will be built.  Instead pass a
variable that try_compile will reference as additional flags.  Leave
this behavior of try_compile undocumented for now.
try_compile: Pass linker flags into test project (#14066)

Copy CMAKE_EXE_LINKER_FLAGS into the test project generated by
try_compile, just like we already copy CMAKE_<LANG>_FLAGS.

Add CMake Policy CMP0056 to activate this behavior in a compatible way,
but do not warn by default when the policy is not set since it will
affect all try_compile calls.

Extend the RunCMake.try_compile test with a case covering this behavior
for each policy setting.
Merge topic 'try_compile-link-flags' into next

88eb5824 try_compile: Pass linker flags into test project (#14066)
a4f9b6f0 CMakeDetermineCompilerABI: Use normal linker flags in ABI project