powerpc: switch to generic sys_execve()/kernel_execve()

the only non-obvious part is that current_pt_regs() is really needed
here - task_pt_regs() is NULL for kernel threads; it's OK for ptrace
uses (the thing task_pt_regs() is intended for), but not for us.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/powerpc/include/asm/unistd.h

@@ -420,6 +420,8 @@
 #define __ARCH_WANT_COMPAT_SYS_RT_SIGSUSPEND
 #define __ARCH_WANT_SYS_NEWFSTATAT
 #endif
+#define __ARCH_WANT_SYS_EXECVE
+#define __ARCH_WANT_KERNEL_EXECVE
 
 /*
  * "Conditional" syscalls