OS X: Use 'uname -m' for processor (#14712)

In commit 2412d9bc (Use 'uname -m' for processor on Cygwin, 2010-09-21)
it was pointed out that POSIX requires 'uname -m' but not 'uname -p':

  http://www.opengroup.org/onlinepubs/9699919799/utilities/uname.html

On OS X x86_64, 'uname -p' returns i386 while 'uname -m' returns x86_64.
Since the latter is the POSIX-compliant option trust its value.
However, on OS X ppc, 'uname -m' returns 'Power Macintosh' so add a
special-case to convert that to 'powerpc'.
Merge topic 'osx-host-arch' into next

9d2a0900 OS X: Use 'uname -m' for processor (#14712)