Add --skipcrossgen to cibuild.sh

In the general case, finding the correct crossgen to invoke is non
trivial since there's no strong corespondence between the host Runtime
ID (which is easy to compute) and the RID that crossgen would be
packaged in.

For example both RHEL and Centos have crossgen packaged in a nuget
packages that uses `rhel.7-x64` as the RID, in order to share the
package across minor versions of RHEL and Centos (which is binary
compatibile).

Since we can build Roslyn without crossgening the bootstrapped
toolchain, I'm exposing an option to allow skipping of crossgen for
platforms where a developer knows apriori that `crossgen.sh` would not
compute the correct RID and fail.

Makefile

@@ -48,8 +48,12 @@ bootstrap: $(TOOLSET_PATH) $(RESTORE_SEMAPHORE_PATH)
 	$(MSBUILD_CMD) src/Compilers/VisualBasic/VbcCore/VbcCore.csproj && \
 	mkdir -p $(BOOTSTRAP_PATH) && \
 	cp -f Binaries/$(BUILD_CONFIGURATION)/Exes/CscCore/* $(BOOTSTRAP_PATH) && \
-	cp -f Binaries/$(BUILD_CONFIGURATION)/Exes/VbcCore/* $(BOOTSTRAP_PATH) && \
-	build/scripts/crossgen.sh $(BOOTSTRAP_PATH) && \
+	cp -f Binaries/$(BUILD_CONFIGURATION)/Exes/VbcCore/* $(BOOTSTRAP_PATH)
+
+ifneq ($(SKIP_CROSSGEN),true)
+	build/scripts/crossgen.sh $(BOOTSTRAP_PATH)
+endif
+
 	rm -rf Binaries/$(BUILD_CONFIGURATION)
 
 test:

cibuild.sh

@@ -7,16 +7,18 @@ usage()
     echo "usage: cibuild.sh [options]"
     echo ""
     echo "Options"
-    echo "  --debug     Build Debug (default)"
-    echo "  --release   Build Release"
-    echo "  --skiptests Do not run tests"
-    echo "  --nocache   Force download of toolsets"
+    echo "  --debug         Build Debug (default)"
+    echo "  --release       Build Release"
+    echo "  --skiptests     Do not run tests"
+    echo "  --skipcrossgen  Do not crossgen the bootstrapped compiler"
+    echo "  --nocache       Force download of toolsets"
 
 }
 
 BUILD_CONFIGURATION=Debug
 USE_CACHE=true
 SKIP_TESTS=false
+SKIP_CROSSGEN=false
 
 MAKE="make"
 if [[ $OSTYPE == *[Bb][Ss][Dd]* ]]; then
@@ -51,6 +53,10 @@ do
         SKIP_TESTS=true
         shift 1
         ;;
+        --skipcrossgen)
+        SKIP_CROSSGEN=true
+        shift 1
+        ;;
         *)
         usage 
         exit 1
@@ -58,7 +64,7 @@ do
     esac
 done
 
-MAKE_ARGS="BUILD_CONFIGURATION=$BUILD_CONFIGURATION"
+MAKE_ARGS="BUILD_CONFIGURATION=$BUILD_CONFIGURATION SKIP_CROSSGEN=$SKIP_CROSSGEN"
 
 if [ "$CLEAN_RUN" == "true" ]; then
     echo Clean out the enlistment