riscv: Improve exception and system call latency

Many CPUs implement return address branch prediction as a stack. The
RISCV architecture refers to this as a return address stack (RAS). If
this gets corrupted then the CPU will mispredict at least one but
potentally many function returns.

There are two issues with the current RISCV exception code:

- We are using the alternate link stack (x5/t0) for the indirect branch
  which makes the hardware think this is a function return. This will
  corrupt the RAS.

- We modify the return address of handle_exception to point to
  ret_from_exception. This will also corrupt the RAS.

Testing the null system call latency before and after the patch:

Visionfive2 (StarFive JH7110 / U74)
baseline: 189.87 ns
patched:  176.76 ns

Lichee pi 4a (T-Head TH1520 / C910)
baseline: 666.58 ns
patched:  636.90 ns

Just over 7% on the U74 and just over 4% on the C910.

Signed-off-by: Anton Blanchard <antonb@tenstorrent.com>
Signed-off-by: Cyril Bur <cyrilbur@tenstorrent.com>
Tested-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20240607061335.2197383-1-cyrilbur@tenstorrent.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>