Fix pgm_read_ptr() (#5735)

Fix dereferencing in pgm_read_ptr() macro on 8266 and host.

Fixes #5733

Author: earlephilhower

tools/sdk/libc/xtensa-lx106-elf/include/sys/pgmspace.h

@@ -73,11 +73,11 @@ static inline uint16_t pgm_read_word_inlined(const void* addr) {
 #ifdef __cplusplus
     #define pgm_read_dword(addr)            (*reinterpret_cast<const uint32_t*>(addr))
     #define pgm_read_float(addr)            (*reinterpret_cast<const float*>(addr))
-    #define pgm_read_ptr(addr)              (*reinterpret_cast<const void*>(addr))
+    #define pgm_read_ptr(addr)              (*reinterpret_cast<const void* const *>(addr))
 #else
     #define pgm_read_dword(addr)            (*(const uint32_t*)(addr))
     #define pgm_read_float(addr)            (*(const float*)(addr))
-    #define pgm_read_ptr(addr)              (*(const void*)(addr))
+    #define pgm_read_ptr(addr)              (*(const void* const*)(addr))
 #endif
 
 #define pgm_read_byte_near(addr)        pgm_read_byte(addr)