[x86] expose __cpuid, __cpuid_count, xgetbv, __readeflags, __writeeflags

Expose the `__cpuid` and `_xgetby` `x86`/`x86_64` intrinsics.

The `__cpuid` and `__cpuid_count` intrinsics are not available on all
`x86` CPUs. The `has_cpuid() -> bool` intrinsic detect this on non
`x86_64` hosts. For convenience, this is exposed on `x86_64` as well
but there it always returns `true`. These are exposed by Clang and
GCC.

The `__readeflags` and `__writeeflags` intrinsics, which read/write
the `EFLAGS` register and are required to implement `has_cpuid`, are
exposed as well. GCC and Clang exposes them too.

When doing run-time feature detection for `x86`/`x86_64` we
now properly check whether the `cpuid` instruction is
available before using it. If it is not available,
are features are exposes as "not available".

One TODO:

- The `_xgetbv` intrinsic requires the `xsave` target feature
but this is not currently exposed by rustc, see #167 .

Author: gnzlbg