MIPS: Prevent link failure with kcov instrumentation

__sanitizer_cov_trace_pc() is not linked in and causing link
failure if KCOV_INSTRUMENT is enabled. Fix this by disabling
instrumentation for compressed image.

Signed-off-by: Jouni Hogander <jouni.hogander@unikie.com>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: linux-mips@vger.kernel.org