spirv-val: Aborts are always packed as scalar (#6859)

@alan-baker sorry to make you bounce around, but @r-potter went to go
fix the GLSL in https://github.com/KhronosGroup/glslang/pull/4398 and
discussed with Tobias that it should be always scalar

source/val/validate_explicit_layout.cpp

diff --git a/source/val/validate_explicit_layout.cpp b/source/val/validate_explicit_layout.cpp
index d9bc87c..8ec2200 100644
--- a/source/val/validate_explicit_layout.cpp
+++ b/source/val/validate_explicit_layout.cpp
@@ -349,11 +349,9 @@
       reference->type_id = inst->GetOperandAs<uint32_t>(0u);
       // Abort messages doesn't have a storage class
       reference->storage_class = spv::StorageClass::Max;
-      // Currently not specified well
-      // https://gitlab.khronos.org/spirv/SPIR-V/-/work_items/962
-      reference->layout = vstate.options()->scalar_block_layout
-                              ? LayoutMode::kScalar
-                              : LayoutMode::kStandard;
+      // The abort message is always packed with scalar rules
+      // See https://gitlab.khronos.org/vulkan/vulkan/-/work_items/4954
+      reference->layout = LayoutMode::kScalar;
       reference->requirement = LayoutRequirement::kRequired;
       return true;
     }