[libc] Allow target architecture independent configs

[sivachandra]

Summary

Previously, we required entrypoints.txt for every target architecture
supported by a target OS. With this change, we allow architecture
independent config for a target OS. That is, if an architecture specific
entrypoints.txt is missing, then a generic entrypoints.txt for that
target OS will be used.

libc/CMakeLists.txt

message(WARNING "
     This is not recommended, to enable set LLVM_LIBC_ENABLE_LINTING to ON
     (pass -DLLVM_LIBC_ENABLE_LINTING=ON to cmake).")
 endif()
 
 include(CMakeParseArguments)
 include(LLVMLibCRules)
 include(LLVMLibCCheckCpuFeatures)
 
-include("${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCHITECTURE}/entrypoints.txt")
+if(EXISTS "${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCHITECTURE}/entrypoints.txt")
+  set(entrypoint_file "${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCHITECTURE}/entrypoints.txt")
+elseif(EXISTS "${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/entrypoints.txt")
+  set(entrypoint_file "${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/entrypoints.txt")
+else()
+  message(FATAL_ERROR "entrypoints.txt file for the target platform not found.")
+endif()
+include(${entrypoint_file})
+
+if(EXISTS "${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCHITECTURE}/headers.txt")
   include("${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCHITECTURE}/headers.txt")
+elseif(EXISTS "${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/headers.txt")
+  include("${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}//headers.txt")
+endif()
 
 set(TARGET_ENTRYPOINT_NAME_LIST "")
 foreach(entrypoint IN LISTS TARGET_LLVMLIBC_ENTRYPOINTS)
   string(FIND ${entrypoint} "." last_dot_loc REVERSE)
   if(${last_dot_loc} EQUAL -1)
     message(FATAL "Invalid entrypoint target name ${entrypoint}; Expected a '.' "
                   "(dot) in the name.")
   endif()