Default to using lld for arm.arm, but disable fatal linker warnings

Since BFD ld 2.17.50 cannot cope with the output of clang 11 and higher,
due to some object files getting more than 65k sections, we need to use
lld 11 and higher instead.

However, this results in warnings from lld: "lld uses blx instruction,
no object with architecture supporting feature detected". At the moment
we think it is safe to ignore this warning, so add a check to bsd.lib.mk
to make them non-fatal by default for arm.arm.