Thank you so much for your help! You’re a lifesaver!
After switching my arm-none-eabi-gcc to the one downloaded from theARM website, I can see the files libm.a and libc.a.
After that I faced an identical issue with @mcrajah (-L issue), but I was able to resolve it thanks to @mcrajah 's previous comments.
I’m just one foot away from succeeding cross-compilation with clang… 
Currently my lld.ld fails due to the symbol COPY in the linker file (nrf52.ld).
I’m not sure what I need to do to fix this one…
$ COMPILER=clang make
error: /home/lonelyjoe/workspace/embedded/interrupt/example/freertos-example-llvm/nrf52.ld:76: symbol not found: COPY
clang: error: ld.lld command failed with exit code 1 (use -v to see invocation)
Makefile:154: recipe for target '/home/lonelyjoe/workspace/embedded/interrupt/example/freertos-example-llvm/build/nrf52.elf' failed
make: *** [/home/lonelyjoe/workspace/embedded/interrupt/example/freertos-example-llvm/build/nrf52.elf] Error 1