tests/tcg/riscv64: Add test for MEPC bit masking

tests/tcg/riscv64: Add test for MEPC bit masking

Add a regression test to verify that MEPC properly masks the lower
bits when an address with mode bits is written to it, as required by
the RISC-V Privileged Architecture specification.

The test sets STVEC to an address with bit 0 set (vectored mode),
triggers an illegal instruction exception, copies STVEC to MEPC in the
trap handler, and verifies that MEPC masks bits [1:0] correctly for
IALIGN=32.

Without the fix, MEPC retains the mode bits (returns non-zero/FAIL).
With the fix, MEPC clears bits [1:0] (returns 0/PASS).

Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250703182157.281320-3-charmitro@posteo.net>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>