Remove the workaround for clang 3.6.2 that did not support
'cortex-m0plus'.
clang 3.8 was already supporting it according to the PR introducing the check.
clang >=3.8 is avaible since ubuntu-xenial and debian-stretch.
The current ubuntu-bionic has clang 6 and debian-buster clang 7.
This removes overwriting 'CPU_ARCH'.
7416337d4d