riscv: Fix toolchain vector detection

A recent change to gcc flags rv64iv as no longer valid:

   cc1: sorry, unimplemented: Currently the 'V' implementation
   requires the 'M' extension

and as a result vector support is disabled. Fix this by adding m
to our toolchain vector detection code.

Signed-off-by: Anton Blanchard <antonb@tenstorrent.com>
Fixes: fa8e7cce ("riscv: Enable Vector code to be built")
Link: https://lore.kernel.org/r/20240819001131.1738806-1-antonb@tenstorrent.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>