malloc: Record caller backtrace for each allocation

When CONFIG_MCHECK_HEAP_PROTECTION is enabled, use backtrace_str() to
capture the caller information for each malloc/calloc/realloc/memalign
call. This information is stored in the mcheck header and can be viewed
with 'malloc dump'.

Add a flag to disable the backtrace when the stack is corrupted, since
the backtrace code tries to walks the invalid stack frames and will
crash.

Note: A few allocations made during libbacktrace initialisation may
not have caller info since they occur during the first backtrace call.

Example output showing caller info:
    18a1d010   90  used  log_init:453 <-board_init_r:774
    18a1d0a0 6060  used  membuf_new:420 <-console_record
    18a3b840   90  used  of_alias_scan:911 <-board_init_

Fix up the backtrace test to avoid recursion.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

doc/usage/cmd/malloc.rst

@@ -59,6 +59,18 @@ Example
     Used: c2ef0 bytes in 931 chunks
     Free: 5f3f0c0 bytes in 2 chunks + top
 
+With CONFIG_MCHECK_HEAP_PROTECTION enabled, the caller backtrace is shown::
+
+    => malloc dump
+    Heap dump: 18a1d000 - 1ea1f000
+         Address        Size  Status
+    ----------------------------------
+        18a1d000          10  (chunk header)
+        18a1d010          90  used  log_init:453 <-board_init_r:774
+        18a1d0a0        6060  used  membuf_new:420 <-console_record
+        18a3b840          90  used  of_alias_scan:911 <-board_init_
+        ...
+
 Configuration
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