Implement stringify macro

And use it for `FW_ARCHITECTURE`. This is needed because with
Microchip's xc32, `-DFOO=\"bar\"` results in `FOO` being `bar`, not
`"bar"`, so we have to stringify differently.

PUBLISHED_FROM=d0aeebb55ef9e0e92a7f750104cd6600e0d9c636

mongoose.h

@@ -1846,6 +1846,23 @@ int cs_base64_decode(const unsigned char *s, int len, char *dst, int *dec_len);
 #define CS_ENABLE_TO64 0
 #endif
 
+/*
+ * Expands to a string representation of its argument: e.g.
+ * `CS_STRINGIFY_LIT(5) expands to "5"`
+ */
+#define CS_STRINGIFY_LIT(x) #x
+
+/*
+ * Expands to a string representation of its argument, which is allowed
+ * to be a macro: e.g.
+ *
+ * #define FOO 123
+ * CS_STRINGIFY_MACRO(FOO)
+ *
+ * expands to 123.
+ */
+#define CS_STRINGIFY_MACRO(x) CS_STRINGIFY_LIT(x)
+
 #ifdef __cplusplus
 extern "C" {
 #endif