Fix FireEtXplatGCBulkRootStaticVar guard to use FEATURE_EVENTSOURCE_XPLAT

[Copilot]

The #else branch for HOST_UNIX unconditionally compiled the FireEtXplatGCBulkRootStaticVar call on all Unix targets, but the PR #128745 changes generated FireEtXplat* declarations to only exist under FEATURE_EVENTSOURCE_XPLAT. Non-browser Unix targets (macOS, Android, WASI) that don't define this feature flag would fail to build.

• Changed guard from bare #else to #elif defined(FEATURE_EVENTSOURCE_XPLAT) with ERROR_SUCCESS fallback when neither Windows ETW nor LTTng/Xplat is available
• Searched all compiled C++ (*.cpp, *.h) in src/coreclr for other FireEtXplat call sites — this is the only one

#if !defined(HOST_UNIX)
    ULONG result = EventWrite(...);
#elif defined(FEATURE_EVENTSOURCE_XPLAT)
    ULONG result = FireEtXplatGCBulkRootStaticVar(...);
#else
    ULONG result = ERROR_SUCCESS;
#endif