syscall: handle undefined r2 value on linux-ppc64x

This change fixes two failng tests on linux-ppc64x:

- TestAllThreadsSyscall() exposed a real bug in the ppc64x support:
  - It turns out that the r2 syscall return value is not defined
    on all architectures. Notably linux-ppc64x so address that by
    introducing a private architectural constant in the syscall
    package, archHonorsR2: true if r2 has a determanistic value.

- TestSetuidEtc() was sensitive to /proc/<PID>/status content:
  - The amount of padding space has changed with kernel vintage.
  - Stress testing revealed a race with /proc files disappearing.

Fixes #42178

Change-Id: Ie6fc0b8f2f94a409ac0e5756e73bfce113274709
Reviewed-on: https://go-review.googlesource.com/c/go/+/266202
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/syscall/syscall_linux_386.go

@@ -6,6 +6,12 @@ package syscall
 
 import "unsafe"
 
+// archHonorsR2 captures the fact that r2 is honored by the
+// runtime.GOARCH.  Syscall conventions are generally r1, r2, err :=
+// syscall(trap, ...).  Not all architectures define r2 in their
+// ABI. See "man syscall".
+const archHonorsR2 = true
+
 const _SYS_setgroups = SYS_SETGROUPS32
 
 func setTimespec(sec, nsec int64) Timespec {