ARMClang: Fix regression in check for working compiler

Given the compiler to use, `CMakeFindBinUtils.cmake` automatically
determines a number of tools including linker (CMAKE_LINKER) and archiver
(CMAKE_AR) and stores them in a generated file `CMakeCCompiler.cmake` as
non-CACHE entries. The compiler-specific ARMClang.cmake then tries to
override CMAKE_LINKER and CMAKE_AR as CACHE entries.

Following the introduction of CMP0126, which is set to NEW in the test
for a working compiler, setting a CACHE entry does not replace a normal
entry of the same name anymore, resulting in a failed test due to wrong
linker and archiver.

To fix this, set CMAKE_LINKER and CMAKE_AR for ARMClang directly in
`CMakeFindBinUtils.cmake` as is done for other compilers.  Check
for them in `ARMClang.cmake` to safeguard cases when a project explicitly
includes `ARMClang.cmake` prior to compiler determination (which some
projects do to work around other problems in older CMake versions).