ARMClang: Fix computation of compiler semantic version patch level

The patch semantic version computation should be updated according to the latest ARMClang documentation 6.20:
https://developer.arm.com/documentation/101754/0620/armclang-Reference/armclang-Command-line-Options/--version-number--armclang-

armclang displays the version number in the format <M><mm><uu><xx>, where:
<M> is the major version number, 6.
<mm> is the minor version number.
<uu> is the update number.
<xx> is reserved for Arm internal use. You can ignore this for the purposes of checking whether the current release is a specific version or within a range of versions.

Backport: release