malloc: Add an option to disable mcheck-backtrace collection

Backtrace collection is relatively expensive and can significantly slow
down malloc()-heavy code when mcheck is enabled.

Add a new CONFIG_MCHECK_BACKTRACE option (default y) to allow disabling
backtrace collection while keeping the other mcheck features (canaries,
double-free detection, etc.) enabled. This allows using mcheck with less
overhead when caller information is not needed.

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

doc/develop/malloc.rst

@@ -130,6 +130,13 @@ Main U-Boot (post-relocation)
     ``malloc dump``. This significantly increases memory overhead and should
     only be used for debugging. Default: n
 
+``CONFIG_MCHECK_BACKTRACE``
+    Bool to enable backtrace collection for mcheck caller information. When
+    enabled (the default), each allocation records a stack backtrace showing
+    where it was made. This is useful for debugging memory leaks but adds
+    overhead to every malloc call. Disable this to reduce mcheck overhead
+    while keeping the canary checks and double-free detection. Default: y
+
 xPL Boot Phases
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