update regarding Mac M1

Author: jsiek

README.md

@@ -9,3 +9,9 @@ The `runtime.c` file needs to be compiled by doing the following
 This will produce a file named `runtime.o`. The -g flag is to tell the
 compiler to produce debug information that you may need to use
 the gdb (or lldb) debugger.
+
+On a Mac with an M1 (ARM) processor, use the `-arch x86_64` flag to
+compile the runtime:
+```
+   gcc -c -g -std=c99 -arch x86_64 runtime.c
+```