Add support for optionally ignoring rmask

[chgentso]

Some cores do not have read mask support for memory accesses, this PR makes it possible to handle them.

[cliffordwolf]

That should not be necessary. Simply set rmaks to all-bits-set. As long as you correctly provide the data for all bits in rdata everything should work just fine.

See the mem_rmask checks in rvfi_insn_check.sv: they only require that the core sets at least the required bits in rmask. It's always correct to read more bytes from memory.