RP2040: add simbols to linker script to compute .uninitialized_data section size

Author: pennam

targets/TARGET_RASPBERRYPI/TARGET_RP2040/TOOLCHAIN_GCC_ARM/memmap_default.ld

@@ -183,7 +183,9 @@ SECTIONS
 
     .uninitialized_data (COPY): {
         . = ALIGN(4);
+        __uninitialized_data_start__ = .;
         *(.uninitialized_data*)
+        __uninitialized_data_end__ = .;
     } > RAM
 
     /* Start and end symbols must be word-aligned */