Switch ARM GCC toolchain to use string_join()
Switch ARM GCC toolchain to use string_join()

This switches the ARM GCC toolchain to use the GN's new string_join()
function to make the long literals in the GCC toolchain files easier to
read.

Change-Id: I7fac49d591a4f589cb11afa41d213adaef411b30

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed