backtrace: sandbox: Add -rdynamic to export symbols

Add the -rdynamic linker flag so that backtrace_symbols() can resolve
function names. This is needed to show meaningful symbol names instead
of just addresses.

Only enable this CONFIG_BACKTRACE and CONFIG_CMDLINE are enabled, since
it causes build failures with the tools-only build and sandbox_nocmdline

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

arch/sandbox/config.mk

@@ -28,6 +28,11 @@ SANITIZERS	+= -fsanitize=fuzzer
 endif
 KBUILD_CFLAGS	+= $(SANITIZERS)
 
+# Avoid defeating linker's garbage collection
+ifeq ($(CONFIG_BACKTRACE)$(CONFIG_CMDLINE),yy)
+RDYNAMIC += -rdynamic
+endif
+
 cmd_u-boot__ = $(CC) -o $@ -Wl,-T u-boot.lds $(u-boot-init) \
 	$(KBUILD_LDFLAGS:%=-Wl,%) \
 	$(SANITIZERS) \
@@ -36,7 +41,7 @@ cmd_u-boot__ = $(CC) -o $@ -Wl,-T u-boot.lds $(u-boot-init) \
 		$(u-boot-main) \
 		$(u-boot-keep-syms-lto) \
 	-Wl,--no-whole-archive \
-	$(PLATFORM_LIBS) -Wl,-Map -Wl,u-boot.map -Wl,--gc-sections
+	$(RDYNAMIC) $(PLATFORM_LIBS) -Wl,-Map -Wl,u-boot.map -Wl,--gc-sections
 
 cmd_u-boot-spl = (cd $(obj) && $(CC) -o $(SPL_BIN) -Wl,-T u-boot-spl.lds \
 	$(KBUILD_LDFLAGS:%=-Wl,%) \