boost: Patch B2 bootstrap step to use correct toolchain (#21408)

var/spack/repos/builtin/packages/boost/bootstrap-toolset.patch

@@ -0,0 +1,11 @@
+--- a/bootstrap.sh	2020-12-03 00:00:59.000000000 -0500
++++ a/bootstrap.sh	2021-01-08 13:38:30.000000000 -0500
+@@ -223,7 +223,7 @@
+ if test "x$BJAM" = x; then
+   $ECHO "Building B2 engine.."
+   pwd=`pwd`
+-  (cd "$my_dir/tools/build/src/engine" && ./build.sh)
++  (cd "$my_dir/tools/build/src/engine" && ./build.sh "$TOOLSET")
+   if [ $? -ne 0 ]; then
+       echo
+       echo "Failed to build B2 build engine"

var/spack/repos/builtin/packages/boost/package.py

@@ -260,6 +260,11 @@ def libs(self):
     # See https://github.com/boostorg/python/pull/218
     patch('boost_218.patch', when='@1.63.0:1.67.99')
 
+    # Fix B2 bootstrap toolset during installation
+    # See https://github.com/spack/spack/issues/20757
+    # and https://github.com/spack/spack/pull/21408
+    patch("bootstrap-toolset.patch", when="@1.75:")
+
     def patch(self):
         # Disable SSSE3 and AVX2 when using the NVIDIA compiler
         if self.spec.satisfies('%nvhpc'):