backtrace: sandbox: Add support for obtaining symbols

Add backtrace functions for sandbox, including:

- os_backtrace() to collect addresses into a caller-supplied buffer
- os_backtrace_symbols() to convert addresses to symbol strings
- os_backtrace_symbols_free() to free the symbol array

The libbacktrace library (bundled with GCC) reads DWARF debug information
to provide detailed symbol resolution including function names (even for
static functions), source file paths, and line numbers.

The sandbox backtrace implementation wraps these OS functions to implement
the generic backtrace API (backtrace_init, backtrace_get_syms, etc.).

Enable it for just the 'sandbox' board. Add the library for the Rust
example too.

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

examples/ulib/config.mk

@@ -44,4 +44,4 @@ SHARED_LDFLAGS := -L$(UBOOT_BUILD) -lu-boot -Wl,-rpath,$(UBOOT_BUILD)
 STATIC_LDFLAGS := -Wl,-T,$(LIB_STATIC_LDS) \
 	-Wl,--whole-archive $(UBOOT_BUILD)/libu-boot.a \
 	-Wl,--no-whole-archive \
-	-lpthread -ldl $(PLATFORM_LIBS) -Wl,-z,noexecstack
+	-lpthread -ldl -lbacktrace $(PLATFORM_LIBS) -Wl,-z,noexecstack