Remove unused Makefile variables FIBER_ASSEMBLER and FIBER_ASM_ARCH

In Windows build system these were replaced with common PHP_ASSEMBLER
and FIBER_ASM_ABI when adjusting for the arm64.

Closes GH-13263

Author: petk

UPGRADING.INTERNALS

@@ -59,6 +59,8 @@ PHP 8.4 INTERNALS UPGRADE NOTES
      system.
    - HAVE_WIN32_NATIVE_THREAD, USE_WIN32_NATIVE_THREAD, ENABLE_THREADS symbols
      in ext/mbstring/libmbfl removed.
+   - FIBER_ASSEMBLER and FIBER_ASM_ARCH Makefile variables removed in favor of
+     PHP_ASSEMBLER and FIBER_ASM_ABI.
 
 ========================
 3. Module changes

win32/build/config.w32

@@ -252,14 +252,6 @@ if (!PHP_ASSEMBLER) {
 	ERROR("No assembler found, fiber cannot be built");
 }
 DEFINE('PHP_ASSEMBLER', PHP_ASSEMBLER);
-DEFINE('FIBER_ASSEMBLER', PHP_ASSEMBLER);// for compatible
-
-var FIBER_ASM_ARCH = {
-	'x64': 'x86_64',
-	'x86': 'i386',
-	'arm64': 'arm64'
-}[TARGET_ARCH];
-DEFINE('FIBER_ASM_ARCH', FIBER_ASM_ARCH); // for compatible only
 
 var FIBER_ASM_ABI = {
 	'x64': 'x86_64_ms_pe_masm',