debug/elf: add C source for testdata executables

This source file, when compiled with gcc 4.4.3 on Ubuntu lucid,
corresponds instruction for instruction to the binaries in the same
directory.

Shipping this source code file resolves http://bugs.debian.org/716853

R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/46780043

src/pkg/debug/elf/testdata/hello.c

+#include <stdio.h>
+
+void
+main(int argc, char *argv[])
+{
+	printf("hello, world\n");
+}