ptrace test fails to build on ARM, struct pt_regs undefined

The ptrace regression test uses struct pt_regs on ARM (and PowerPC), but doesn't appear to include the necessary headers to define that struct:

$ gcc -o ptrace ptrace.c
ptrace.c: In function ‘read_ptrace_registers’:
ptrace.c:58:19: error: storage size of ‘regs’ isn’t known
  ARCH_REGS_STRUCT regs;
                   ^

On ARM, the right one appears to be <asm/ptrace.h>, unless you know better. With the attached patch, it works.

The code path where struct pt_regs is used is if "defined(__arm__) || defined(__powerpc__) || defined(__powerpc64__)", but that struct seems to be pulled in by one of the headers that's already used on both the PowerPCs (possibly <sys/ptrace.h>).

Build-tested on Debian armhf, powerpc, ppc64.