Update docs and scripts to make arm64 a synonym for aarch64 (chap…

…el-lang#22049)

[reviewed and curated by @ronawho and @mppf]

This updates the description of the `CHPL_HOST_ARCH` options to include
`arm64`, which is what we infer it to be on Mac M1/M2 platforms, and a
string that other UNIX instances reportedly use as well.

It also updates our `chplenv` scripts that look for `aarch64` to make
them recognize `arm64` as follows:
* updates a check in `chpl_platform.py` to treat `arm64` similarly to
`aarch64`
* generalizes a check in `chpl_cpu.py` to look for `arm` as a prefix
rather than `arm-` since this would handle the `arm64` case
* updates a comment in `chpl_cpu.py` to name `arm64` as a possible
`uname[4]` output as well

I chose not to update a final `aarch64` check in `chpl_cpu.py` because
it seemed to assume that it was a valid platform value, which I'm not
aware that it is, so didn't feel confident that it should be updated
(and also didn't feel confident enough that it's dead code to remove
it).

doc/rst/usingchapel/chplenv.rst

@@ -146,6 +146,7 @@ CHPL_HOST_ARCH
         ========  =============================================================
         x86_64    64-bit AMD and Intel processors
         aarch64   64-bit ARM processors
+        arm64     an alternative name for 'aarch64'
         ========  =============================================================
 
    If unset, the default will be computed. The command ``uname -m``

util/chplenv/chpl_cpu.py

@@ -29,7 +29,7 @@
   'x86-trento':      'znver3',
 }
 
-# This gets the generic machine type, e.g. x86_64, i686, aarch64.
+# This gets the generic machine type, e.g. x86_64, i686, aarch64, arm64.
 # Since uname returns the host machine type, we currently assume that
 # cross-compilation is limited to different subarchitectures of the
 # generic machine type.  For example, we can cross compile from
@@ -40,7 +40,7 @@ def get_native_machine():
 
 @memoize
 def is_known_arm(cpu):
-    if cpu.startswith("arm-") or ('aarch64' in cpu) or ('thunderx' in cpu):
+    if cpu.startswith("arm") or ('aarch64' in cpu) or ('thunderx' in cpu):
         return True
     else:
         return False

util/chplenv/chpl_platform.py

@@ -58,7 +58,7 @@ def get(flag='host'):
                     platform_val = "linux64_32"
                 else:
                     platform_val = "linux64"
-            elif machine == 'aarch64':
+            elif machine == 'aarch64' or machine == 'arm64':
                 platform_val = "linux64"
             else:
                 platform_val = "linux32"