pw_toolchain: Support grouping in generate_toolchain
pw_toolchain: Support grouping in generate_toolchain

This allows linking projects with circular dependencies without
requiring -Wl,--whole-archive.

Change-Id: I230edca9a0f5d2c922455ce3bb258ae089ed09e0

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