[m-rev.] for review: document Linux AArch64 support

[Julien Fischer]

For review by anyone.

I've bootchecked Mercury fairly extensively on a Graviton 2 based EC2
instance and haven't encountered any major issues with the exception of
the debugging and profdeep grades where the EC2 instance doesn't have
enough memory :-(  (I have bootchecked a debug grade on my Chromebook
and will do so for a profdeep grade.)

---------------------------------------

Document Linux AArch64 support.

NEWS:
     As above.

.README.in:
     Add pointer the to the Linux/AArch64 specific README file.

     Delete a left-over reference to the Alpha port.

README.Linux:
     Add pointer the to the Linux/AArch64 specific README file.

README.Linux-aarch64:
     Add a README file specific to the Linux/AArch64 port.

Julien.

diff --git a/.README.in b/.README.in
index 464c4e6ce..5f8873f06 100644
--- a/.README.in
+++ b/.README.in
@@ -12,7 +12,7 @@ platform, if there is one:
  	README.FreeBSD		FreeBSD on Intel x86 and x86_64
  	README.HPUX		HPPA systems running HPUX
  	README.Linux		Linux on Intel x86 and x86_64
-	README.Linux-Alpha	Linux on DEC Alpha
+	README.Linux-aarch64    Linux on AArch64
  	README.Linux-PPC	Linux on PowerPC
  	README.Linux-m68k	Linux on Motorola 68000
  	README.MacOS		Apple Mac systems running Mac OS X
diff --git a/NEWS b/NEWS
index fcc42bb20..af70e7bdc 100644
--- a/NEWS
+++ b/NEWS
@@ -304,6 +304,11 @@ Changes to the Mercury compiler
    keep `opt1` enabled even if `opt1` is not normally enabled at optimization
    level `N`.

+Portability improvements
+------------------------
+
+* We have ported Mercury to Linux AArch64 (ARM64) systems.
+
  Changes to the Mercury debugger
  -------------------------------

diff --git a/README.Linux b/README.Linux
index 456350bed..3c1ce83e7 100644
--- a/README.Linux
+++ b/README.Linux
@@ -1,4 +1,5 @@
  This file documents the Linux/x86 and Linux/x86_64 ports.
+For the Linux/AArch64 port, see REAMDE.Linux-aarch64.
  For the Linux/PowerPC port, see README.Linux-PPC.
  (Ports to Linux on other architectures have not yet
  been attempted, but should not be difficult.)
diff --git a/README.Linux-aarch64 b/README.Linux-aarch64
index e69de29bb..90bf86a06 100644
--- a/README.Linux-aarch64
+++ b/README.Linux-aarch64
@@ -0,0 +1,4 @@
+This file documents the port of Mercury to Linux on AArch64 (ARM64),
+i.e. the `aarch64*-linux*` configuration.
+
+Mercury should build and install "out-of-the-box" on Linux for AArch64.