break build if it would produce broken binary

Add an error in known-bad case so that we don't produce broken and
hard to debug binaries.

Signed-off-by: Pavel Machek <pavel@denx.de>

arch/arm/include/asm/u-boot.h

@@ -49,4 +49,8 @@ typedef struct bd_info {
 #define IH_ARCH_DEFAULT IH_ARCH_ARM64
 #endif
 
+#if defined(CONFIG_USE_PRIVATE_LIBGCC) && defined(CONFIG_SYS_THUMB_BUILD)
+#error Thumb build does not work with private libgcc.
+#endif
+
 #endif	/* _U_BOOT_H_ */