crossgcc/Makefile: clean more, add distclean target

Update the clean target to remove the intermediate files.  These should
get removed automatically, but if the build stops in the middle, or if
the -t command is used for buildgcc, they can be left in the directory.

Add a distclean target that removes the downloaded tarballs as well as
everything else.

Change-Id: I6ea19e7a499b0c313c1d2eff7e36386204ec834e
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: http://review.coreboot.org/10621
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

util/crossgcc/Makefile

@@ -44,5 +44,13 @@ build-riscv-without-gdb:
 
 clean:
 	rm -rf xgcc
+	rm -rf build-*
+	rm -rf binutils-* gcc-* gmp-* libelf-* mpc-* mpfr-*
+	rm -rf llvm-* clang-tools-* cfe-* compiler-rt-*
+	rm -rf acpica-*
+	rm -rf gdb-*
 
-.PHONY: all build clean
+distclean: clean
+	rm -rf tarballs
+
+.PHONY: all build clean distclean