u3: switch page size constants to macros to avoid VLAs
static u3p(c3_w) gar_pag_p;
//! Urbit page size in 4-byte words.
-static const size_t pag_wiz_i = (size_t)1 << u3a_page;
+#define pag_wiz_i ((size_t)1 << u3a_page)
//! Urbit page size in bytes.
-static const size_t pag_siz_i = (size_t)1 << (u3a_page + 2);
+#define pag_siz_i ((size_t)1 << (u3a_page + 2))
#ifdef U3_SNAPSHOT_VALIDATION
/* Image check.
