arm64: barrier: Restore spec_bar() macro

Upcoming errata workarounds will need to use SB from C code. Restore the
spec_bar() macro so that we can use SB.

This is effectively a revert of commit:

  4f30ba1c ("arm64: barrier: Remove spec_bar() macro")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20240508081400.235362-2-mark.rutland@arm.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/barrier.h

@@ -40,6 +40,10 @@
  */
 #define dgh()		asm volatile("hint #6" : : : "memory")
 
+#define spec_bar()	asm volatile(ALTERNATIVE("dsb nsh\nisb\n",		\
+						 SB_BARRIER_INSN"nop\n",	\
+						 ARM64_HAS_SB))
+
 #ifdef CONFIG_ARM64_PSEUDO_NMI
 #define pmr_sync()						\
 	do {							\