Use $ARMC5_BIN_DIR or --armc5-bin-dir to set the location of
armcc (version 5), for compatibility with later versions of Mbed TLS.
Ignore --armc6-bin-dir for the same reason, since in 2.1 we only test
with a single version of armcc and we expect version 5. Keep
defaulting to armcc in the $PATH for backward compatibility with the
2.1 branch.
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