fix(build): restore baseline WebKit suffix for x64 builds

Fixes #26635

The fix from #26071 was accidentally reverted in commit b268004715
("Upgrade WebKit to d5bd162d9ab2"). This re-applies the fix to request
the baseline WebKit variant for x64 baseline builds.

Without this, baseline x64 builds use a WebKit compiled with
-march=haswell (AVX/AVX2), causing illegal instruction (SIGILL) crashes
when running under QEMU emulation or on older CPUs without AVX support.

Uses the already-computed WEBKIT_ARCH variable instead of duplicating
the architecture regex pattern.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmake/tools/SetupWebKit.cmake

@@ -87,6 +87,11 @@ if(LINUX AND ABI STREQUAL "musl")
   set(WEBKIT_SUFFIX "-musl")
 endif()
 
+# Baseline builds require a WebKit compiled without AVX instructions
+if(ENABLE_BASELINE AND WEBKIT_ARCH STREQUAL "amd64")
+  set(WEBKIT_SUFFIX "${WEBKIT_SUFFIX}-baseline")
+endif()
+
 if(DEBUG)
   set(WEBKIT_SUFFIX "${WEBKIT_SUFFIX}-debug")
 elseif(ENABLE_LTO)