fix(cmake): download baseline WebKit for x64 baseline builds

When ENABLE_BASELINE is set for x64 builds, Bun is compiled with
-march=nehalem (no AVX/AVX2), but the prebuilt WebKit was downloaded
without a -baseline suffix, linking against a WebKit compiled with
AVX/AVX2 support. This caused SIGILL crashes on CPUs without AVX.

Add -baseline suffix to the WebKit download URL when ENABLE_BASELINE
is true for amd64 targets, matching the baseline artifact names from
oven-sh/WebKit PR #137.

Also remove unused WEBKIT_PREVIEW_PR variable (PR #140 was merged).

Closes #27006

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

cmake/tools/SetupWebKit.cmake

@@ -9,9 +9,6 @@ if(NOT WEBKIT_VERSION)
   set(WEBKIT_VERSION 8af7958ff0e2a4787569edf64641a1ae7cfe074a)
 endif()
 
-# Use preview build URL for Windows ARM64 until the fix is merged to main
-set(WEBKIT_PREVIEW_PR 140)
-
 string(SUBSTRING ${WEBKIT_VERSION} 0 16 WEBKIT_VERSION_PREFIX)
 string(SUBSTRING ${WEBKIT_VERSION} 0 8 WEBKIT_VERSION_SHORT)
 
@@ -209,6 +206,12 @@ if(LINUX AND ABI STREQUAL "musl")
   set(WEBKIT_SUFFIX "-musl")
 endif()
 
+# Baseline builds target older CPUs without AVX/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)