x86_64: Build kernel with debug info, like all other archs

The defconfigs for all architectures other than x86_64 has this
enabled, so align it with the rest.

This has no on-target impact, but it enables us to attach to a live VM
instance and debug it, or do offline debugging like resolving stack
trace lines from oops messages to source locations.

Author: wkz

board/x86_64/linux_defconfig

@@ -261,6 +261,7 @@ CONFIG_NLS_ISO8859_1=y
 CONFIG_CRYPTO_AES=y
 CONFIG_CRYPTO_GCM=y
 CONFIG_DEBUG_KERNEL=y
+CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
 CONFIG_MAGIC_SYSRQ=y
 CONFIG_DEBUG_FS=y
 CONFIG_PANIC_ON_OOPS=y