riscv: Enable ARCH_HAVE_NMI_SAFE_CMPXCHG

riscv: Enable ARCH_HAVE_NMI_SAFE_CMPXCHG

The implement of cmpxchg() in riscv is based on atomic primitives and
has NMI-safe features, so it can be used safely in the in_nmi context.
ftrace's ringbuffer relies on NMI-safe cmpxchg() in the NMI context.
Currently, in_nmi() is true when riscv kprobe is in trap-based mode, so
this config needs to be selected, otherwise kprobetrace will not be
available.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250711090443.1688404-1-pulehui@huaweicloud.com
[pjw@kernel.org: moved to preserve alphabetical order]
Signed-off-by: Paul Walmsley <pjw@kernel.org>