In triggers, use optional<data> instead of {has_data, data}

Simplify handling of load/store/fetch slow-path cases; fix two minor trigger bugs

h/t @YenHaoChen

Add --dm-no-abstract-fpr option.

The shift amount is 6 bits wide on RV64.  As with the base ISA shifts, we
ignore XLEN and unconditionally disassemble the 6-bit immediate on RV32.

Partially reverts da93bdc4

Previously FPRs could always be accessed using abstract commands. I need
this to get coverage of some OpenOCD code that I broke. (See
https://github.com/riscv/riscv-openocd/pull/745)

The scheme was based on the notion that memory accesses are idempotent
up until the point the trigger would've been hit, which isn't true in
the case of side-effecting loads and data-value triggers.

Instead, check the trigger on the next instruction fetch.  To keep the
perf overhead minimal, perform this check on the I$ refill path, and
ensure that path is taken by flushing the I$.

Instruction fetch is always little-endian.

As a side effect, misaligned stores now behave the same as aligned stores
with respect to triggers: only the first byte is checked.

As a side effect, misaligned loads now behave the same as aligned loads
with respect to triggers: only the first byte is checked.

Fix trigger priority

Fix or work around ignored-qualifiers, unused-function, unused-parameter, and unused-variable warnings

Signed-off-by: Jerin Joy <joy@rivosinc.com>

Suppressing these individually would add too much clutter.