Compiler option -no-plt will break kernel builds on some architectures
eg. (x86)  Filter this option from the recently introduced handling of
KCFLAGS vs EXTRA_OPTIMISATION

Fixes: 1d42af72 ("kernel: use KCFLAGS for passing EXTRA_OPTIMIZATION flags")

Suggested-by: Felix Fietkau <nbd@nbd.name>

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>