powerpc: Move EXCEPTION_TABLE to RO_DATA segment

Since the EXCEPTION_TABLE is read-only, collapse it into RO_DATA.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-alpha@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-c6x-dev@linux-c6x.org
Cc: linux-ia64@vger.kernel.org
Cc: linux-s390@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Will Deacon <will@kernel.org>
Cc: x86-ml <x86@kernel.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Link: https://lkml.kernel.org/r/20191029211351.13243-25-keescook@chromium.org

arch/powerpc/kernel/vmlinux.lds.S

@@ -7,6 +7,7 @@
 
 #define BSS_FIRST_SECTIONS *(.bss.prominit)
 #define EMITS_PT_NOTE
+#define RO_EXCEPTION_TABLE_ALIGN	0
 
 #include <asm/page.h>
 #include <asm-generic/vmlinux.lds.h>
@@ -162,7 +163,6 @@ SECTIONS
 		__stop__btb_flush_fixup = .;
 	}
 #endif
-	EXCEPTION_TABLE(0)
 
 /*
  * Init sections discarded at runtime