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>