arm64 value for <instruction-set> feature support

[dmigous@…]

I want to build boost for arm64 instruction set. But getting error:

error: "arm64" is not a known value of feature <instruction-set>

This error occurs because builtin.jam file contains just following valid values for instruction-set feature:

armv2 armv2a armv3 armv3m armv4 armv4t armv5 armv5t armv5te armv6 armv6j armv6j iwmmxt ep9312 armv7 armv7s

I also checked several versions of boost - all of them don't contain arm64 value at builtin.jam file. Meanwhile arm64 is a valid argument to -arch option for compiler (e.g. clang)

So my question is: why don't add there "arm64" value to the list of valid values for instruction-set feature?