crossgcc: Add gnumake target so that make can be built directly

Previously, make could be built as one of the crosgcc* targets, but
there was no way to just rebuild make, as there is for IASL.

- Add an independent target - gnumake.
- Add gnumake to the help text.
- Add gnumake to the list of NOCOMPILE targets (Not compiling coreboot)

Change-Id: I4df25f2e209ca14944d491dbfb8e9b085ff7aca3
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/16163
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Makefile

@@ -122,7 +122,7 @@ ifeq ($(strip $(HAVE_DOTCONFIG)),)
 NOCOMPILE:=1
 endif
 ifneq ($(MAKECMDGOALS),)
-ifneq ($(filter %config %clean cross% clang iasl lint% help% what-jenkins-does,$(MAKECMDGOALS)),)
+ifneq ($(filter %config %clean cross% clang iasl gnumake lint% help% what-jenkins-does,$(MAKECMDGOALS)),)
 NOCOMPILE:=1
 endif
 ifeq ($(MAKECMDGOALS), %clean)