Add MSYS2 target

This patch enables the use of QBE's experimental winabi target. This
target is found on the winabi branch of QBE, at c9x.me/git/qbe.git.

configure

@@ -122,6 +122,10 @@ case "$target" in
 	endfiles='"-l", "c", "-l", ":crtn.o"'
 	defines='"-D", "__builtin_stdarg_start(ap, last)=__builtin_va_start(ap, last)"'
 	;;
+*-*msys*)
+	startfiles='"-l", ":crt0.o"'
+	endfiles='"-l", "c", "-l", "msys-2.0", "-l", "kernel32"'
+	;;
 *)
 	fail "unknown target '$target', please create config.h manually"
 esac

driver.c

@@ -384,8 +384,13 @@ main(int argc, char *argv[])
 	arrayaddbuf(&stages[LINK].cmd, linkcmd, sizeof(linkcmd));
 
 	if (hasprefix(target, "x86_64-") || hasprefix(target, "amd64-")) {
-		arch = "x86_64-sysv";
-		qbearch = "amd64_sysv";
+		if (strstr(target, "msys")) {
+			arch = "x86_64-sysv";
+			qbearch = "amd64_win";
+		} else {
+			arch = "x86_64-sysv";
+			qbearch = "amd64_sysv";
+		}
 	} else if (hasprefix(target, "aarch64-")) {
 		arch = "aarch64";
 		qbearch = "arm64";