target/riscv: Replace watchpoint value mask comparison value with macro.

This patch replaces ~(typeof(watchpoint->mask))0 with
WATCHPOINT_IGNORE_DATA_VALUE_MASK. This improves
readability and moves the RISCV target in line with
other targets.

Change-Id: I15ac4d4ee76098b304d9b22f720911ba4329c190
Signed-off-by: Marek Vrbka <marek.vrbka@codasip.com>