x86/boot/64: Fix spurious undefined reference when CONFIG_X86_5LEVEL=n, on GCC-12

In __startup_64(), the bool 'la57' can only assume the 'true' value if
CONFIG_X86_5LEVEL is enabled in the build, and generally, the compiler
can make this inference at build time, and elide any references to the
symbol 'level4_kernel_pgt', which may be undefined if 'la57' is false.

As it turns out, GCC 12 gets this wrong sometimes, and gives up with a
build error:

   ld: arch/x86/kernel/head64.o: in function `__startup_64':
   head64.c:(.head.text+0xbd): undefined reference to `level4_kernel_pgt'

even though the reference is in unreachable code. Fix this by
duplicating the IS_ENABLED(CONFIG_X86_5LEVEL) in the conditional that
tests the value of 'la57'.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241209094105.762857-2-ardb+git@google.com
Closes: https://lore.kernel.org/oe-kbuild-all/202412060403.efD8Kgb7-lkp@intel.com/

Author: ardbiesheuvel

arch/x86/kernel/head64.c

@@ -186,7 +186,7 @@ unsigned long __head __startup_64(unsigned long p2v_offset,
 	pgd = &RIP_REL_REF(early_top_pgt)->pgd;
 	pgd[pgd_index(__START_KERNEL_map)] += load_delta;
 
-	if (la57) {
+	if (IS_ENABLED(CONFIG_X86_5LEVEL) && la57) {
 		p4d = (p4dval_t *)&RIP_REL_REF(level4_kernel_pgt);
 		p4d[MAX_PTRS_PER_P4D - 1] += load_delta;