arm64: Reserve HWCAP2_MTE as (1 << 18)

While MTE is not supported in the upstream kernel yet, add a comment
that HWCAP2_MTE as (1 << 18) is reserved. Glibc makes use of it for the
resolving (ifunc) of the MTE-safe string routines.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/hwcap.h

@@ -95,6 +95,7 @@
 #define KERNEL_HWCAP_DGH		__khwcap2_feature(DGH)
 #define KERNEL_HWCAP_RNG		__khwcap2_feature(RNG)
 #define KERNEL_HWCAP_BTI		__khwcap2_feature(BTI)
+/* reserved for KERNEL_HWCAP_MTE	__khwcap2_feature(MTE) */
 
 /*
  * This yields a mask that user programs can use to figure out what

arch/arm64/include/uapi/asm/hwcap.h

@@ -74,5 +74,6 @@
 #define HWCAP2_DGH		(1 << 15)
 #define HWCAP2_RNG		(1 << 16)
 #define HWCAP2_BTI		(1 << 17)
+/* reserved for HWCAP2_MTE	(1 << 18) */
 
 #endif /* _UAPI__ASM_HWCAP_H */

arch/arm64/kernel/cpuinfo.c

@@ -93,6 +93,7 @@ static const char *const hwcap_str[] = {
 	"dgh",
 	"rng",
 	"bti",
+	/* reserved for "mte" */
 	NULL
 };