uapi/auxvec: Define AT_HWCAP3 and AT_HWCAP4 aux vector, entries

The powerpc toolchain keeps a copy of the HWCAP bit masks in the TCB
for fast access by the __builtin_cpu_supports() built-in function. The
TCB space for the HWCAP entries - which are created in pairs - is an ABI
extension, so waiting to create the space for HWCAP3 and HWCAP4 until
they are needed is problematic. Define AT_HWCAP3 and AT_HWCAP4 in the
generic uapi header so they can be used in glibc to reserve space in the
powerpc TCB for their future use.

I scanned through the Linux and GLIBC source codes looking for unused
AT_* values and 29 and 30 did not seem to be used, so they are what I
went with.

Signed-off-by: Peter Bergner <[e-mail]>
Acked-by: Adhemerval Zanella <[e-mail]>
Acked-by: Nicholas Piggin <[e-mail]>
Acked-by: Szabolcs Nagy <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

include/uapi/linux/auxvec.h

···
 #define AT_HWCAP2 26	/* extension of AT_HWCAP */
 #define AT_RSEQ_FEATURE_SIZE	27	/* rseq supported feature size */
 #define AT_RSEQ_ALIGN		28	/* rseq allocation alignment */
+#define AT_HWCAP3 29	/* extension of AT_HWCAP */
+#define AT_HWCAP4 30	/* extension of AT_HWCAP */
 
 #define AT_EXECFN  31	/* filename of program */