target/riscv: fix access permission checks for CSR_SSP

target/riscv: fix access permission checks for CSR_SSP

Commit:8205bc1 ("target/riscv: introduce ssp and enabling controls for
zicfiss") introduced CSR_SSP but it mis-interpreted the spec on access
to CSR_SSP in M-mode. Gated to CSR_SSP is not gated via `xSSE`. But
rather rules clearly specified in section "22.2.1. Shadow Stack Pointer
(ssp) CSR access contr" in the priv spec.

Fixes: 8205bc127a83 ("target/riscv: introduce ssp and enabling controls
for zicfiss". Thanks to Adam Zabrocki for bringing this to attention.

Reported-by: Adam Zabrocki <azabrocki@nvidia.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250306064636.452396-1-debug@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
(cherry picked from commit 86c78b280607fcff787866a03374047c65037a90)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>