isa_validator: nullify Svnapot in march strings

the get_march_mabi function should return a string that
can be consumed by the riscv-gcc right away. the compiler
complains about the presence of svnapot in the said string.
"nullify" this extension, resulting in exclusion during march generation.

Signed-off-by: Karthik B K <karthik.bk@incoresemi.com>

riscv_config/isa_validator.py

@@ -210,6 +210,9 @@ def get_march_mabi (isa : str, opt_remove_custom_exts: bool = False):
         'Zbf',
         'Zbm',
         'Zbr',
+
+        # supervisor address translations
+        'Svnapot',
     ]
 
     # add Zbp and Zbt to null_ext if Zbpbo is present