Mcause::from_bits doesn't allow interrupt to be set. #333
Description
Mcause::from_bits is documented as only setting BITMASK bits, but this is set to 0xffff_ffff. This excludes the interrupt bit on rv64, at bit 63. It should include at least this bit, but probably all 64 bits, since the riscv privileged spec says the Exception Code field is MXLEN-1 (with Interrupt at MXLEN-1)