pkg/debian: Disable options that prevent building kernel modules with older GCCs

Disable some options that prevent building external kernel modules with
older GCC versions.

pkg/debian/kernel/ubuntu.config

@@ -78,3 +78,12 @@ CONFIG_I2C_DESIGNWARE_BAYTRAIL=y
 CONFIG_I2C_DESIGNWARE_PCI=y
 CONFIG_INTEL_SOC_PMIC=y
 CONFIG_PWM_CRC=y
+
+##
+## Disable some mitigations that aren't available on older gcc versions. This
+## is necessary to allow older gcc versions to be used for building kernel
+## modules.
+##
+
+# CONFIG_SLS is not set
+# CONFIG_ZERO_CALL_USED_REGS is not set