powerpc longjmp: Fix build after chk hidden builtin fix

04bf7d2d ("chk: Add and fix hidden builtin definitions for *_chk")
added an #undef for longjmp and siglongjmp to compensate for the
definition in include/setjmp.h, but missed doing so for the powerpc
version too.

Fixes: 04bf7d2d ("chk: Add and fix hidden builtin definitions for
*_chk")