arm64: vdso: Fix population of AT_SYSINFO_EHDR for compat vdso

Prior to the introduction of Unified vDSO support and compat layer for
vDSO on arm64, AT_SYSINFO_EHDR was not defined for compat tasks.
In the current implementation, AT_SYSINFO_EHDR is defined even if the
compat vdso layer is not built, which has been shown to break Android
applications using bionic:

 | 01-01 01:22:14.097   755   755 F libc    : Fatal signal 11 (SIGSEGV),
 | code 1 (SEGV_MAPERR), fault addr 0x3cf2c96c in tid 755 (cameraserver),
 | pid 755 (cameraserver)
 | 01-01 01:22:14.112   759   759 F libc    : Fatal signal 11 (SIGSEGV),
 | code 1 (SEGV_MAPERR), fault addr 0x3cf2c96c in tid 759
 | (android.hardwar), pid 759 (android.hardwar)
 | 01-01 01:22:14.120   756   756 F libc    : Fatal signal 11 (SIGSEGV)
 | code 1 (SEGV_MAPERR), fault addr 0x3cf2c96c in tid 756 (drmserver),
 | pid 756 (drmserver)

Restore the old behaviour by making sure that AT_SYSINFO_EHDR for compat
tasks is defined only when CONFIG_COMPAT_VDSO is enabled.
Reported-by: John Stultz <john.stultz@linaro.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/elf.h

@@ -202,7 +202,7 @@ typedef compat_elf_greg_t		compat_elf_gregset_t[COMPAT_ELF_NGREG];
 ({									\
 	set_thread_flag(TIF_32BIT);					\
  })
-#ifdef CONFIG_GENERIC_COMPAT_VDSO
+#ifdef CONFIG_COMPAT_VDSO
 #define COMPAT_ARCH_DLINFO						\
 do {									\
 	/*								\