Disable interception of sigaltstack on i386 macOS.

Summary:
28c91219 introduced an interceptor for `sigaltstack`. It turns out this
broke `setjmp` on i386 macOS. This is because the implementation of `setjmp` on
i386 macOS is written in assembly and makes the assumption that the call to
`sigaltstack` does not clobber any registers.  Presumably that assumption was
made because it's a system call.  In particular `setjmp`  assumes that before
and after the call that `%ecx` will contain a pointer the `jmp_buf`. The
current interceptor breaks this assumption because it's written in C++ and
`%ecx` is not a callee-saved register. This could be fixed by writing a
trampoline interceptor to the existing interceptor in assembly that
ensures all the registers are preserved. However, this is a lot of work
for very little gain. Instead this patch just disables the interceptor
on i386 macOS.

For other Darwin architectures it currently appears to be safe to intercept
`sigaltstack` using the current implementation because:

* `setjmp` for x86_64 saves the pointer `jmp_buf` to the stack before calling `sigaltstack`.
* `setjmp` for armv7/arm64/arm64_32/arm64e appears to not call `sigaltstack` at all.

This patch should unbreak (once they are re-enabled) the following
tests:

```
AddressSanitizer-Unit :: ./Asan-i386-calls-Test/AddressSanitizer.LongJmpTest
AddressSanitizer-Unit :: ./Asan-i386-calls-Test/AddressSanitizer.SigLongJmpTest
AddressSanitizer-Unit :: ./Asan-i386-inline-Test/AddressSanitizer.LongJmpTest
AddressSanitizer-Unit :: ./Asan-i386-inline-Test/AddressSanitizer.SigLongJmpTest
AddressSanitizer-i386-darwin :: TestCases/longjmp.cpp
```

This patch introduces a `SANITIZER_I386` macro for convenience.

rdar://problem/62141412

Reviewers: kubamracek, yln, eugenis

Subscribers: kristof.beyls, #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D82691