riscv: use lw when reading int cpu in asm_per_cpu

riscv: use lw when reading int cpu in asm_per_cpu

REG_L is wrong, because thread_info.cpu is 32-bit, not xlen-bit wide.
The struct currently has a hole after cpu, so little endian accesses
seemed fine.

Fixes: be97d0db5f44 ("riscv: VMAP_STACK overflow detection thread-safe")
Cc: stable@vger.kernel.org
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Radim Krčmář <rkrcmar@ventanamicro.com>
Link: https://lore.kernel.org/r/20250725165410.2896641-5-rkrcmar@ventanamicro.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>