Predefine __ppc and __ppc__ for VxWorks 7

Predefine __ppc and __ppc__ for VxWorks 7

Unfortunately, some VxWorks 7r2 system headers rely on a
couple more variations of the predefined macros expected
to characterize a "powerpc" target that we discussed recently.

setjmp.h, for example, relies on __ppc and the absence of
a definition results in "gcc" dejagnu test failures from all
the tests #including that header, which stumble on:

  #error "_JBLEN not set!"

The other case is __ppc__ expected by yvals.h, key to libstdc++.

This change adjusts the VxWorks 7 section of our configuration
to honor those expectations.

2020-10-29  Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/rs6000/vxworks.h (TARGET_OS_CPP_BUILTINS): Also
	builtin_define __ppc and __ppc__ for VxWorks 7.