riscv: Compile with march

Tell the compiler to provide mnemonics for instructions we depend on,
such as 'pause'. Specifying march also allows extensions which affect
compilation to be individually [un]selected. For example, building
without compressed (2 byte) instructions may be desirable, so 'c' may
be removed from the march isa string.

Signed-off-by: Andrew Jones <andrew.jones@linux.dev>
Acked-by: Thomas Huth <thuth@redhat.com>