MdePkg: Enable new MS VA intrinsics for GNUC x86 64bits build
MdePkg: Enable new MS VA intrinsics for GNUC x86 64bits build

Both GCC and LLVM 3.8 64bits support new variable argument (VA)
intrinsics for Microsoft ABI, enable these new VA intrinsics for
GNUC family 64bits code build. These VA intrinsics are only
permitted use in 64bits code, so not use them in 32bits code build.
The original 32bits GNU VA intrinsics has the same calling convention
as MS, so we don't need change them.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Steven Shi <steven.shi@intel.com>

[ardb: update CPP logic so that the change only applies to X64]
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>
Reviewed-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Tested-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>
Tested-By: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Reviewed-by: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>

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