MdePkg/BaseLib: add PatchInstructionX86()

Some edk2 modules generate X86 machine code at module execution time by:

- compiling "template" code with NASM at module build time,

- linking the object code into the module,

- and patching the immediate (constant) operands of some instructions when
  the module is executed.

Add a helper function to BaseLib so that the C code performing the
patching is easier to read and maintain.

The implementation in this patch is taken mainly from Mike Kinney's
mailing list messages at
<http://mid.mail-archive.com/E92EE9817A31E24EB0585FDF735412F5B895C360@ORSMSX113.amr.corp.intel.com>,
<http://mid.mail-archive.com/E92EE9817A31E24EB0585FDF735412F5B898BF66@ORSMSX112.amr.corp.intel.com>.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=866
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Library/BaseLib/BaseLib.inf

@@ -431,6 +431,7 @@
   X86DisablePaging64.c
   X86DisablePaging32.c
   X86RdRand.c
+  X86PatchInstruction.c
 
 [Sources.X64]
   X64/Thunk16.nasm
@@ -757,6 +758,7 @@
   X86DisablePaging64.c
   X86DisablePaging32.c
   X86RdRand.c
+  X86PatchInstruction.c
   X64/GccInline.c | GCC
   X64/Thunk16.S | XCODE 
   X64/SwitchStack.nasm| GCC