fix: use baseline WebKit for x64 baseline builds to avoid SIGILL on pre-AVX2 CPUs

When ENABLE_BASELINE is set for x64 builds, the Bun executable is correctly
compiled with -march=nehalem (no AVX/AVX2), but the WebKit library was
downloaded without a -baseline suffix, pulling a build that may contain
AVX/AVX2 instructions. This causes SIGILL crashes on older Intel CPUs
(e.g. Xeon E5-2450, Sandy Bridge/Ivy Bridge) that lack AVX2 support.

Add -baseline suffix to WEBKIT_SUFFIX when ENABLE_BASELINE is set for amd64,
so baseline builds download bun-webkit-*-amd64-baseline.tar.gz instead.

Note: This requires corresponding baseline WebKit builds to be published from
the oven-sh/WebKit repository (see oven-sh/WebKit#151).

Closes #26872

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

cmake/tools/SetupWebKit.cmake

@@ -209,6 +209,12 @@ if(LINUX AND ABI STREQUAL "musl")
   set(WEBKIT_SUFFIX "-musl")
 endif()
 
+# Baseline builds (ENABLE_BASELINE) target older CPUs without AVX2 (e.g. nehalem).
+# They require a WebKit library also compiled without AVX/AVX2 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)