Makefile: fix empty MK_ARCH when using ccache

Makefile: fix empty MK_ARCH when using ccache

One can use ccache by prefixing the typical CROSS_COMPILE value with
"ccache " (e.g. "ccache aarch64-gnu-linux-" for Aarch64). This however
makes the MK_ARCH empty because sed won't find a match anymore since it
expects the CROSS_COMPILE value to start with the actual toolchain (with
an unlimited number of white spaces before).

This is failing builds since commit 7506c1566998 ("sandbox: Report host
default-filename in native mode").

Add "ccache" prefix to ignore but participate in the matching regex used
by sed to identify the target architecture.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>