ARM64: Add GCS support in assembly code

The GCS support is properly enabled as seen with the following command:

$ readelf -n build/libleancrypto.so

Displaying notes found in: .note.gnu.property
  Owner                Data size        Description
  GNU                  0x00000010       NT_GNU_PROPERTY_TYPE_0
      Properties: AArch64 feature: BTI, PAC, GCS

Signed-off-by: Stephan Mueller <smueller@chronox.de>

Author: smuellerDD

CHANGES.md

@@ -7,6 +7,8 @@ Changes 1.6.1-prerelease
 
 * Fix some side channel test failures (all failures are due to test case issues, and no real problems)
 
+* AARCH64: enable GCS support (see https://community.arm.com/arm-community-blogs/b/tools-software-ides-blog/posts/gcc-15-continuously-improving#guarded and https://docs.kernel.org/next/arch/arm64/gcs.html)
+
 Changes 1.6.0
 * ASN.1: use stack for small generator for small use cases
 

internal/api/assembler_support.h

@@ -73,11 +73,21 @@
 #  define GNU_PROPERTY_AARCH64_POINTER_AUTH 0
 # endif
 
+# if defined(__ARM_FEATURE_GCS_DEFAULT) && __ARM_FEATURE_GCS_DEFAULT == 1
+#  define GNU_PROPERTY_AARCH64_GCS (1<<2)
+# else
+#  define GNU_PROPERTY_AARCH64_GCS 0 /* No GCS */
+# endif
+
 /* Add the BTI / PAC support to GNU Notes section */
-# if GNU_PROPERTY_AARCH64_BTI != 0 || GNU_PROPERTY_AARCH64_POINTER_AUTH != 0
+# if GNU_PROPERTY_AARCH64_BTI != 0 ||                                          \
+     GNU_PROPERTY_AARCH64_POINTER_AUTH != 0 ||                                 \
+     GNU_PROPERTY_AARCH64_GCS != 0
 #  define LC_GNU_PROPERTY_ALIGN 3 /* 2 on 32 bit systems, which we do not cover here */
 #  define LC_GNU_PROPERTY_TYPE 0xc0000000
-#  define LC_GNU_PROPERTY_DATA (GNU_PROPERTY_AARCH64_BTI|GNU_PROPERTY_AARCH64_POINTER_AUTH)
+#  define LC_GNU_PROPERTY_DATA (GNU_PROPERTY_AARCH64_BTI |                     \
+                                GNU_PROPERTY_AARCH64_POINTER_AUTH |            \
+                                GNU_PROPERTY_AARCH64_GCS)
 # endif
 #endif