arch-arm: Add a message to a static_assert in isa.hh.

static_asserts without a message are a c++17 feature.

Change-Id: I9d8b5f5a0d7f9b83784f0480afab0f534a466ee5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44386
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/arch/arm/isa.hh

@@ -856,7 +856,8 @@ namespace ArmISA
         static void
         zeroSveVecRegUpperPart(Elem *v, unsigned eCount)
         {
-            static_assert(sizeof(Elem) <= sizeof(uint64_t));
+            static_assert(sizeof(Elem) <= sizeof(uint64_t),
+                    "Elem type is too large.");
             eCount *= (sizeof(uint64_t) / sizeof(Elem));
             for (int i = 16 / sizeof(Elem); i < eCount; ++i) {
                 v[i] = 0;