riscv: Drop mstrict-align

The spec says unaligned accesses are supported, so this isn't required
and clang doesn't support it. A platform might have slow unaligned
accesses, but kvm-unit-tests isn't about speed anyway.

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