RFC: Enable __declspec for Linux/x86

[H. J. Lu]

Many x86 SSE source codes use __declspec. I'd like to make
__declspec available for Linux/x86. We can do one of the
following:

1. Define TARGET_DECLSPEC for Linux/x86.
2. Define TARGET_DECLSPEC for x86.
3. Add -mdeclspec.

Any comments?


H.J.