Convert another README to Markdown.

Documentation/README.x86.md:
   As above.

   Move the the bit about x86-64 not being affected by the issue
   described in this file from the bottom to the top.

Documentation/README.Linux.md:
   Add a pointer to the README.x86.md file.

README.md:
   Conform to the above change.

Documentation/README.x86.md

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+Mercury on x86
+--------------
+
+This file contains a note for users who are installing Mercury on x86 systems
+that use ELF shared libraries. Note that the x86-64 is *not* affected by
+the issue described below, only x86.
+
+Due to improvements in GCC, the following combination is not supported:
+
+- GCC version 5 and above
+- x86
+- PIC (position-independent code)
+- Low-level C grades using non-local gotos, i.e. `asm_fast.*`
+
+The configure script will not select `asm_fast` grades when the above
+combination is detected. If you try to use that combination by selecting an
+`asm_fast` grade manually, then you will encounter compilation errors.
+
+We recommend using high-level C grades (`hlc.*`) on x86. If you require a
+low-level C grade, e.g. for debugging, then you can use a `reg.*` grade.
+
+If you must use an `asm_fast` grade, then you will need to tell the C compiler
+not to generate position-independent code (which may be the default), *and*
+disable building of shared libraries. Alternatively, you could downgrade to
+GCC version 4.9.
+
+Note that Windows is unaffected by this issue as Windows does not use PIC,
+and we do not yet support shared libraries (DLLs) on Windows anyway.
+
+See <https://bugs.mercurylang.org/view.php?id=453> for further details.