[GCC toolchain builder] Provide means of throttling parallel builds.

Created by: zchamski

Changed files:

• README.md (Build Model>Prerequisites): Describe use of NUM_JOBS for building the toolchain.
• util/gcc-toolchain-builder/build-toolchain.sh (NUM_JOBS): New. (build binutils-gdb): Use NUM_JOBS to throttle parallel MAKE operation. (build gcc): Ditto. (build newlib): Ditto.