tests: cross-compiling hello binaries for hello_se tests.

Some of the hello_se tests fail due to different syntax of the string
for different isas. This patch adds makefiles for cross-compiling the
hello.c file located at tests/test-progs/hello/src/.

Change-Id: I8ccfc0487020df9da722a97e57310db2d2e8882c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28528
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/test-progs/hello/src/Makefile.mips

@@ -0,0 +1,11 @@
+all: hello
+
+hello: hello.c dockcross-mips
+	./dockcross-mips bash -c '$$CC hello.c -o hello -static'
+
+dockcross-mips:
+	docker run --rm dockcross/linux-mips > ./dockcross-mips
+	chmod +x ./dockcross-mips
+
+clean:
+	rm -f dockcross* hello