build: add support for building tools targeting host machine

tools/e64patch/main.c

@@ -0,0 +1,37 @@
+#include <stdio.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include "elf.h"
+
+int main(int argc, const char **argv)
+{
+	if (argc < 2) {
+		fprintf(stderr, "no elf file specified.\n");
+		return -1;
+	}
+
+	unsigned long pagesz = sysconf(_SC_PAGESIZE);
+
+	FILE *fp = fopen(argv[1], "r+b");
+	if (!fp) {
+		perror(argv[1]);
+		return -1;
+	}
+
+	void *p = mmap(NULL, pagesz, PROT_READ | PROT_WRITE, MAP_SHARED, fileno(fp), 0);
+	if (p == MAP_FAILED) {
+		perror("mmap");
+		fclose(fp);
+		return -1;
+	}
+
+	elf_ehdr_t *elf = p;
+	elf->e_machine = EM_386;
+
+	munmap(p, pagesz);
+	fclose(fp);
+
+	printf("patched %s e_machine to EM_386\n", argv[1]);
+	
+	return 0;
+}