Fix StackPointer assembly for ARM64 Windows only

Add specific case for CPU(ARM64) && OS(WINDOWS) that removes the
.previous directive which is not supported by LLVM ARM64 assembler
on Windows. Keep .previous for all other platforms where it's needed.

Fixes: <inline asm>(7,1): error: unknown directive .previous

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: WebKit ARM64 Builder

Source/WTF/wtf/StackPointer.cpp

@@ -96,6 +96,17 @@ asm (
     ".previous" "\n"
 );
 
+#elif CPU(ARM64) && OS(WINDOWS)
+asm (
+    ".text" "\n"
+    ".balign 16" "\n"
+    ".globl " SYMBOL_STRING(currentStackPointer) "\n"
+    SYMBOL_STRING(currentStackPointer) ":" "\n"
+
+    "mov x0, sp" "\n"
+    "ret" "\n"
+);
+
 #elif CPU(ARM64)
 asm (
     ".text" "\n"