cpu: reset byte_enable across writeMem calls

cpu: reset byte_enable across writeMem calls

data_write_req byteEnable which is used in ARM SVE partial writes was not
being zeroed between writes.

As a result, non-SVE memory write instructions such as STP that followed
SVE memory write instructions could still have the write mask active.

This could lead to wrong simulation behaviour, and to an assertion failure:

src/mem/packet.hh:1211: void Packet::writeData(uint8_t*) const: Assertion
`req->getByteEnable().size() == getSize()' failed. '`

Change-Id: I74b5a82675e9923b0ffdf2c1dd9afb00c91cb204
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20448
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>