MdePkg: move to 'hidden' visibility for all symbols under GCC/X64

When using GCC to build for X64, we switched to the position independent
small code model, which is much more efficient in terms of code generation
and runtime relocation footprint, and produces binaries that can execute
correctly from any offset.

However, the PIC routines are by default geared towards hosted binaries
containing symbol references that may resolve to definitions in other
dynamic objects, and for this reason, most symbol references are indirected
via a GOT entry (which also results in a .reloc fixup entry) unless we
annotate them.

For this reason, we introduced the 'protected' visibility annotation for
all symbol definitions and references, by setting the GCC visibility
pragma. However, as it turns out, this is not sufficient for all versions
of GCC, and in some cases (GCC 5.x using the GCC49 toolchain tag), may
still result in GOT based relocations.

So switch to 'hidden' visibility instead, which is slightly stronger, and
fixes this issue for the versions of GCC that exhibit the problem.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>

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