MdePkg X64: force 'protected' visibility when building with -fpic

When building position independent (PIC) ELF objects, the GCC compiler
assumes that each symbol with external linkage may potentially end up
being exported from a shared library, which means that each of those
symbols may be subject to symbol preemption, i.e., the executable
linking to the shared library at runtime may override symbols exported
by the shared library, and every internal reference held by the shared
library itself *must* be made to point to the overridden version instead.

For this reason, PIC code symbol references always go via the Global
Offset Table (GOT), even if the code in question references symbols that
are defined in the same compilation unit. The GOT refers to each symbol
by absolute address, and so each entry is subject to runtime relocation.

Since not every symbol with external linkage is ultimately exported from
a shared library, the GCC compiler allows control over symbol visibility
using attributes, command line arguments and pragmas, where 'protected'
means that the symbol is only referenced by the shared library itself.
Due to the poor hygiene in EDK2 regarding the use of the 'static'
modifier, many symbols that are local to their compilation unit end up
being referenced indirectly via the GOT when building PIC code.

In UEFI, there are no shared libraries and so there is no need to deal
with symbol preemption, and we can mark every symbol reference protected.
The only method that applies to all symbol definitions as well as
declarations is the #pragma. So set the visibility 'protected' pragma when
building PIC code for X64 using GCC. Note that this affects code generated
with the -fpie compiler switch as well as the -fpic compiler switch.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-By: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

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