[clang] [Clang][Sanitizers] Enable NSAN on X86_64 only (PR #95885)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> ~90% of the functionality would work on aarch64, but there are a few x86_64-specific quirks (__float128, x86-64-specific assumptions about long double in the helper routines) that need to be refactored (beyond my current bandwidth).

I am happy to investigate them. I guess we don't need to exclude driver now? xray compiler instrumentation works for Mach-O but the runtime is not yet, we don't disable the driver for Mach-O

https://github.com/llvm/llvm-project/pull/95885