riscv: select DCACHE_WORD_ACCESS for efficient unaligned access HW

DCACHE_WORD_ACCESS uses the word-at-a-time API for optimised string
comparisons in the vfs layer.

This patch implements support for load_unaligned_zeropad in much the
same way as has been done for arm64.

Here is the test program and step:

$ cat tt.c
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

#define ITERATIONS 1000000

#define PATH "123456781234567812345678123456781"

int main(void)
{
unsigned long i;
struct stat buf;

for (i = 0; i < ITERATIONS; i++)
stat(PATH, &buf);

return 0;
}

$ gcc -O2 tt.c
$ touch 123456781234567812345678123456781
$ time ./a.out

Per my test on T-HEAD C910 platforms, the above test performance is
improved by about 7.5%.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20231225044207.3821-3-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

riscv: mm: stub extable related functions/macros for !MMU

extable relies on the MMU to work properly, so it's useless to
include __ex_table sections and build extable related functions for
!MMU case.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20230509152641.805-1-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

riscv: extable: add a dedicated uaccess handler

Inspired by commit 2e77a62cb3a6 ("arm64: extable: add a dedicated
uaccess handler"), do similar to riscv to add a dedicated uaccess
exception handler to update registers in exception context and
subsequently return back into the function which faulted, so we remove
the need for fixups specialized to each faulting instruction.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

riscv: extable: add `type` and `data` fields

This is a riscv port of commit d6e2cc564775 ("arm64: extable: add `type`
and `data` fields").

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

riscv: extable: consolidate definitions

This is a riscv port of commit 819771cc2892 ("arm64: extable:
consolidate definitions").

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>