[DeviceRTL] Fix incremental build

Need both add_custom_command to resolve file-level dependency and add_custom_target to resolve target-level dependency.
From CMake add_custom_command doc:
Do not list the output in more than one independent target that may build in parallel or the two instances of the rule may conflict (instead use the add_custom_target() command to drive the command and make the other targets depend on that one).

${CMAKE_CURRENT_BINARY_DIR}/${bclib_name} is used by multiple targets and thus requires a custom target to avoid racing.

Differential Revision: https://reviews.llvm.org/D136911