build: export potentially overridden tool chain components

build: export potentially overridden tool chain components

When overriding the tool chain via CROSS_COMPILE, the resulting
components need to be made available to, in particular (but not limited
to) the check-endbr.sh script. Note that we don't allow overriding
ADDR2LINE yet; this would first require additions to some config/*.mk
before it would make sense to export the resulting variable as well.

The lack of NM exporting was apparently not a problem so far, but add it
at this occasion as well - we're using the tool, after all.

Fixes: 4d037425dccf ("x86: Build check for embedded endbr64 instructions")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>