MdePkg/X64/ProcessorBind.h: Fix EmulatorPkg X64 XCODE5

MdePkg/X64/ProcessorBind.h: Fix EmulatorPkg X64 XCODE5

https://bugzilla.tianocore.org/show_bug.cgi?id=2046

Make sure all libs that should be dynamically linked
are dynamically linked when XCODE5 compiler is used.
A previous change introduced the following pragma:

    #pragma GCC visibility push (hidden)

When the EmulatorPkg is built using XCODE5 toolchain
for X64, this pragma is included and it generates a
linker error when building the Unix Host module.  This
change uses !defined(__APPLE__) to prevent the use of
this pragma when building EmulatorPkg/Unix/Host.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>