Makefile.cpio_strip: Use "gnu11" instead of deprecated "gnu1x" identifier.
Makefile.cpio_strip: Use "gnu11" instead of deprecated "gnu1x" identifier.

In particular, this causes issues when running Goanna that does not recognise
gnu1x.

winksaville/common-tool,winksaville/common-tool,hendersa/px4-common-tool,hendersa/px4-common-tool,seL4/common-tool,winksaville/common-tool,hendersa/px4-common-tool,seL4/common-tool,seL4/common-tool