boards: riscv: qemu: enable coverage support for all targets

Support code coverage on all RISC-V qemu targets. Additional
boards may choose to enable coveage support as well.

Tested by following the procedure documented at
https://docs.zephyrproject.org/latest/develop/test/coverage.html


with the qemu_riscv64 target.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>