ICU-23269 Add PAC/BTI tags to generated aarch64 assembler data

Background: PAC and BTI are branch and pointer authentication technologies
for aarch64:

https://developer.arm.com/community/arm-community-blogs/b/architectures-and-processors-blog/posts/enabling-pac-and-bti-on-aarch64

For PAC/BTI to be used at runtime, all of the ELF segments loaded need
to confirm that they're compatible.  If given the right options both GCC
and Clang automatically generate code that is compatible and add the
annotations.

However, if data files are built into binary objects with tools/gen*
using assembler, then these tags are not present and all of ICU cannot
use branch protection or pointer authentication.

As the generated assembler is literally only data, we can just add the
tags. This is similar to how Intel CET is enabled in the same file.

Author: rossburton

icu4c/source/tools/toolutil/aarch64.h

@@ -0,0 +1,22 @@
+// Copyright (C) 2025 and later: Unicode, Inc. and others.
+// License & terms of use: http://www.unicode.org/copyright.html
+
+/* Add BTI/PAC/GCS tags to GNU Notes section */
+#if defined(__ARM_FEATURE_BTI_DEFAULT) && __ARM_FEATURE_BTI_DEFAULT == 1
+    #define GNU_PROPERTY_AARCH64_FEATURE_1_AND      0xc0000000
+    #define GNU_PROPERTY_AARCH64_FEATURE_1_BTI      (1U << 0)
+    #define GNU_PROPERTY_AARCH64_FEATURE_1_PAC      (1U << 1)
+    #define GNU_PROPERTY_AARCH64_FEATURE_1_GCS      (1U << 2)
+
+    .pushsection .note.gnu.property, "a"; /* Start a new allocatable section */
+    .balign 8; /* align it on a byte boundry */
+    .long 4; /* size of "GNU\0" */
+    .long 0x10; /* size of descriptor */
+    .long 0x5; /* NT_GNU_PROPERTY_TYPE_0 */
+    .asciz "GNU";
+    .long GNU_PROPERTY_AARCH64_FEATURE_1_AND;
+    .long 4; /* Four bytes of data */
+    .long GNU_PROPERTY_AARCH64_FEATURE_1_BTI | GNU_PROPERTY_AARCH64_FEATURE_1_PAC | GNU_PROPERTY_AARCH64_FEATURE_1_GCS;
+    .long 0; /* padding for 8 byte alignment */
+    .popsection; /* end the section */
+#endif

icu4c/source/tools/toolutil/pkg_genc.cpp

@@ -137,6 +137,9 @@ static const struct AssemblyType {
         "#ifdef __CET__\n"
         "# include <cet.h>\n"
         "#endif\n"
+        "#ifdef __aarch64__\n"
+        "#include \"../tools/toolutil/aarch64.h\"\n"
+        "#endif\n"
         "\t.section .rodata\n"
         "\t.balign 16\n"
         "#ifdef U_HIDE_DATA_SYMBOL\n"