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.Linux.md

@@ -12,8 +12,10 @@ For the Linux/PowerPC port, see
 For the Linux/Motorola 68000 port, see
 [README.Linux-m68k.md](README.Linux-m68k.md).
 
-(Ports to Linux on other architectures have not yet
-been attempted, but should not be difficult.)
+(Ports to Linux on other architectures have not yet been attempted, but should
+not be difficult.)
+
+If you are installing on x86, then also see [README.x86.md](README.x86.md).
 
 Building a 32-bit Mercury Compiler on 64-bit Linux
 --------------------------------------------------

Documentation/README.x86

@@ -1,24 +0,0 @@
-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, 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, 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 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.
-
-Also note x86-64 is not affected by this issue, only x86.
-
-<https://bugs.mercurylang.org/view.php?id=453>

Documentation/README.x86.md

@@ -0,0 +1,30 @@
+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.

README.md

@@ -59,7 +59,7 @@ Specific information is contained in individual README files in the
   * Other platform information
       * [Cross compilation](Documentation/README.cross.md)
       * [Docker](Documentation/README.Docker)
-      * [x86](Documentation/README.x86)
+      * [x86](Documentation/README.x86.md)
 
 ## Other information