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

* 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.

README.md

@@ -163,6 +163,14 @@ located under `util/gcc-toolchain-builder`.  Please make sure beforehand that yo
 installed all the required *toolchain build* dependencies (see
 [the toolchain README file](file:util/gcc-toolchain-builder/README.md).)
 
+To control the load of your host machine when building the toolchain, you can use
+the environment variable `NUM_JOBS` to limit the number of concurrent jobs launched
+by `make`:
+- if left undefined, `NUM_JOBS` will default to 1, resulting in a sequential execution
+of `make` jobs;
+- when setting `NUM_JOBS` to an explicit value, it is recommended not to exceed 2/3 of
+the total nuber of virtual cores available on your system.
+
 ```sh
 # Set environment variable RISCV to the desired installation location.
 # The toolchain can be installed in any user-writable directory.

util/gcc-toolchain-builder/build-toolchain.sh

@@ -37,6 +37,12 @@ ROOT_DIR=$(dirname $(readlink -f $0))
 # Builds of individual tools are performed under the matching
 # build/... subdirectories
 
+# Provide a means of throttling parallel 'make' executions.
+# Use a conservative setting to avoid overloading the host machine.
+if [ -z "${NUM_JOBS}" ]; then
+    NUM_JOBS=1
+fi
+
 # Helper function to print usage information.
 print_usage()
 {
@@ -163,7 +169,7 @@ cd $ROOT_DIR/build/binutils-gdb
 	      { echo "Could not configure binutils-gdb, bailing out!" ; \
 		exit 2 ; } ; } && \
     { [ -d gas/doc ] || mkdir -p gas/doc; } && \
-    make -j all && make install || \
+    make -j${NUM_JOBS} all && make install || \
 	{ echo "*** Could not build binutils, bailing out!" ; exit 2; }
 cd -
 
@@ -182,8 +188,8 @@ cd $ROOT_DIR/build/gcc
 	  ../../$GCC_DIR/configure $GCC_CONFIGURE_OPTS || \
 	      { echo "Could not configure GCC, bailing out!" ; \
 		exit 2 ; } ; } && \
-	make -j all || { rm -rf $TARGET && \
-			     make -j all ; } && make install || \
+	make -j${NUM_JOBS} all || { rm -rf $TARGET && \
+			     make -j${NUM_JOBS} all ; } && make install || \
 	{ echo "*** Could not build GCC (even after removing target dirs), bailing out!" ; exit 2; }
 cd -
 
@@ -209,7 +215,7 @@ export CFLAGS="-mcmodel=medium"
 	  ../../$NEWLIB_DIR/configure $NEWLIB_CONFIGURE_OPTS || \
 	      { echo "Could not configure newlib, bailing out!" ; \
 		exit 2 ; } ; } && \
-    make -j all && make install || \
+    make -j${NUM_JOBS} all && make install || \
 	{ echo "*** Could not build newlib, bailing out!" ; exit 2; }
 cd -