Define new value for the __ARM_ACLE macro (ARM-software#294)

The ACLE defines a feature macro called __ARM_ACLE:

__ARM_ACLE is defined to the version of this specification implemented,
as 100 * major_version + minor_version. An implementation implementing
version 2.1 of the ACLE specification will define __ARM_ACLE as 201.

However, a long time ago, we changed the versioning scheme of the ACLE
from "major.minor" to a combination of year and quarter. For instance
the latest release is 2023 Q2.

Since the macro's definition is now out of date, we now move to a new
scheme:

`__ARM_ACLE` is defined to the version of this specification implemented,
formatted as `{YEAR}{QUARTER}{PATCH}`.

The value would be the integer formed by the year (4 digits)
concatenated to the quarter's number (1 digit), finally concatenated to
the patch release (1 digit). So the macro's value for the 2023 Q2
release with no patch release would then be 202320. Using 4 digits for
the year, and having it preceding the quarter, will be helpful to
ensure the formed integer values will be greater than the ones using the
previous/current format.

main/acle.md

@@ -353,6 +353,8 @@ Armv8.4-A [[ARMARMv84]](#ARMARMv84). Support is added for the Dot Product intrin
 
 #### Changes for next release
 
+* Changed the definition of the `__ARM_ACLE` macro to reflect the current
+  versioning scheme.
 * Combined the SME `slice_base` and `slice_offset` arguments into a
   single `slice` argument.
 * Added the [Keyword attributes](#keyword-attributes) section.
@@ -1327,10 +1329,23 @@ enclose them in parentheses if they are not simple constants.
 
 ## Testing for Arm C Language Extensions
 
-`__ARM_ACLE` is defined to the version of this specification
-implemented, as `100 * major_version + minor_version`. An implementation
-implementing version 2.1 of the ACLE specification will define
-`__ARM_ACLE` as 201.
+`__ARM_ACLE` is defined as the version of this specification that is
+implemented, formatted as `{YEAR}{QUARTER}{PATCH}`. The `YEAR` segment is
+composed of 4 digits, the `QUARTER` segment is composed of 1 digit, and
+the `PATCH` segment is also composed of 1 digit.
+
+For example:
+
+ - An implementation based on the version 2023 Q2 of the ACLE with no
+   further patch releases will define `__ARM_ACLE` as `202320`.
+ - An implementation based on a hypothetical version 2024 Q3 of the ACLE
+   with two patch releases will define `__ARM_ACLE` as `202432`.
+
+NOTE: Previously, the macro followed the previous versioning scheme and
+was defined as `100 * major_version + minor_version`, which was the
+version of this specification implemented. For instance, an implementation
+implementing version 2.1 of the ACLE specification defined `__ARM_ACLE`
+as `201`.
 
 ## Endianness