buildman: Use specific exception type in _fixup_arch()

Change bare except to catch AttributeError specifically, which occurs
when the ARCH_RV32I symbol is not found and get() returns None

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

tools/buildman/boards.py

@@ -322,7 +322,7 @@ class KconfigScanner:
         if params['arch'] == 'riscv':
             try:
                 value = self._conf.syms.get('ARCH_RV32I').str_value
-            except:
+            except AttributeError:
                 value = ''
             if value == 'y':
                 params['arch'] = 'riscv32'