powerpc: Add attributes for setjmp/longjmp

commit aa497d43 upstream.

The setjmp function should be declared as "returns_twice", or bad
things can happen[1]. This does not actually change generated code in
my testing.

The longjmp function should be declared as "noreturn", so that the
compiler can optimise calls to it better. This makes the generated
code a little shorter.

1: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-returns_005ftwice-function-attributeSigned-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c02ce4a573f3bac907e2c70957a2d1275f910013.1567605586.git.segher@kernel.crashing.orgSigned-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/powerpc/include/asm/setjmp.h

@@ -12,7 +12,7 @@
 
 #define JMP_BUF_LEN    23
 
-extern long setjmp(long *);
-extern void longjmp(long *, long);
+extern long setjmp(long *) __attribute__((returns_twice));
+extern void longjmp(long *, long) __attribute__((noreturn));
 
 #endif /* _ASM_POWERPC_SETJMP_H */