When LLVM_TARGETS_TO_BUILD does not contain the host target, runtime
build can not identify a compatible target triple for the host compiler.
CMAKE_C_COMPILER is set to clang, and CMAKE_C_COMPILER_ID is empty
although the compiler is functional.
Remove CMAKE_C_COMPILER_ID check. CMakeTestCLCCompiler.cmake already
provides a real functional guard.
This change ensures valid configurations can proceed, e.g.:
LLVM_TARGETS_TO_BUILD=AMDGPU;
RUNTIMES_amdgcn-amd-amdhsa-llvm_LLVM_ENABLE_RUNTIMES=libclc;
LLVM_RUNTIME_TARGETS="amdgcn-amd-amdhsa-llvm".
Note CMAKE_C_COMPILER_WORKS is always true after `project(Runtimes C CXX
ASM)` in runtimes/CMakeLists.txt.
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