[sparkplug] remove UNREACHABLE from constexpr functions

gcc may throw the following compilation error if UNREACHABLE
is used within a constexpr function:
```
error: call to non-'constexpr' function
                  'void V8_Fatal(const char*, ...)'
```

Bug: v8:11420
Change-Id: I7f8237d00ba1a5d9bd778d45eb833b89cbe8eb24
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2906032
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74688}

src/builtins/builtins.h

@@ -111,7 +111,6 @@ class Builtins {
       case SaveFPRegsMode::kSave:
         return Builtins::kEphemeronKeyBarrierSaveFP;
     }
-    UNREACHABLE();
   }
 
   // Convenience wrappers.

src/wasm/wasm-code-manager.h

@@ -169,7 +169,6 @@ class V8_EXPORT_PRIVATE WasmCode final {
             return RuntimeStubId::kRecordWriteOmitRememberedSetSaveFP;
         }
     }
-    UNREACHABLE();
   }
 
   Vector<byte> instructions() const {