rp2: Mark gc_heap NOLOAD for faster boot.

Create a new linker section .unitialized_bss for bss that does not need
zero-initialising.

Move gc_heap to this section, which saves ~30ms from rising edge of RESET
to setting a pin HIGH in MicroPython.

Zero fill happens in Pico SDK crt0.S before ROSC is configured.  It's very,
very slow.

Signed-off-by: Phil Howard <phil@gadgetoid.com>

ports/rp2/main.c

@@ -66,7 +66,7 @@
 #endif
 
 extern uint8_t __StackTop, __StackBottom;
-static char gc_heap[MICROPY_GC_HEAP_SIZE];
+__attribute__((section(".uninitialized_bss"))) static char gc_heap[MICROPY_GC_HEAP_SIZE];
 
 // Embed version info in the binary in machine readable form
 bi_decl(bi_program_version_string(MICROPY_GIT_TAG));

ports/rp2/memmap_mp.ld

@@ -180,6 +180,12 @@ SECTIONS
         *(.uninitialized_data*)
     } > RAM
 
+    /* bss without zero init on startup */
+    .uninitialized_bss (NOLOAD): {
+        . = ALIGN(4);
+        *(.uninitialized_bss*)
+    } > RAM
+
     /* Start and end symbols must be word-aligned */
     .scratch_x : {
         __scratch_x_start__ = .;