MIPS: Only use current_stack_pointer on GCC

Unfortunately, Clang did not have support for "sp" as a global register
definition, and was crashing after the addition of current_stack_pointer.
This has been fixed in Clang 14, but earlier Clang versions need to
avoid this code, so add a versioned test and revert back to the
open-coded asm instances. Fixes Clang build error:

fatal error: error in backend: Invalid register name global variable

Fixes: 200ed341b864 ("mips: Implement "current_stack_pointer"")
Reported-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]-3990X
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: Guenter Roeck <[e-mail]>
Cc: Yanteng Si <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/Kconfig

···
 	default y
 	select ARCH_32BIT_OFF_T if !64BIT
 	select ARCH_BINFMT_ELF_STATE if MIPS_FP_SUPPORT
-	select ARCH_HAS_CURRENT_STACK_POINTER
+	select ARCH_HAS_CURRENT_STACK_POINTER if !CC_IS_CLANG || CLANG_VERSION >= 140000
 	select ARCH_HAS_DEBUG_VIRTUAL if !64BIT
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KCOV

arch/mips/include/asm/thread_info.h

···
 	return __current_thread_info;
 }
 
+#ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER
 register unsigned long current_stack_pointer __asm__("sp");
+#endif
 
 #endif /* !__ASSEMBLY__ */
 

arch/mips/kernel/irq.c

···
 #ifdef CONFIG_DEBUG_STACKOVERFLOW
 static inline void check_stack_overflow(void)
 {
-	unsigned long sp = current_stack_pointer;
+	unsigned long sp;
 
+	__asm__ __volatile__("move %0, $sp" : "=r" (sp));
 	sp &= THREAD_MASK;
 
 	/*

arch/mips/lib/uncached.c

···
 	register long ret __asm__("$2");
 	long lfunc = (long)func, ufunc;
 	long usp;
-	long sp = current_stack_pointer;
+	long sp;
+
+	__asm__("move %0, $sp" : "=r" (sp));
 
 	if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
 		usp = CKSEG1ADDR(sp);