x86: re-order clang no integrated assembler tests

x86: re-order clang no integrated assembler tests

The tests to check whether the integrated assembler is capable of
building Xen should be performed before testing any assembler
features, or else the feature specific tests would be stale if the
integrated assembler is disabled afterwards.

Fixes: ef286f67787a ('x86: move and fix clang .skip check')
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reported-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>