SIGILL when running the tests with a std built in debug mode

[antoyo]

Hi.
It seems some issues are happening when running the tests with a std built in debug mode.
Run this command to reproduce the issue:

TARGET=x86_64-unknown-linux-gnu cargo test --target x86_64-unknown-linux-gnu -Zbuild-std

In my case, it ends with:

test core_arch::x86::sse41::tests::test_mm_testz_si128 ... error: test failed, to rerun pass `-p core_arch --lib`

Caused by:
  process didn't exit successfully: `stdarch/target/x86_64-unknown-linux-gnu/debug/deps/core_arch-0c16e18ffd10ec9f` (signal: 4, SIGILL: illegal instruction)

I believe other tests ought to fail. For instance, here, we create an empty slice and we get the address of the first element here and there's an assert in debug mode to forbid this.