rs6000: fixinc: Skip machine_name fix for powerpc*-*-linux*

Some system headers can be broken by the machine_name fix performed
by GCC during the fixincludes step. According to the comment in
fixincludes/fixinc.h:130 :

   On some platforms, machine_name doesn't work properly and
   breaks some of the header files.  Since everything works
   properly without it, just wipe the macro list to
   disable the fix.

So we can just skip it to avoid trouble.

	Backport from trunk
	2020-02-13  Matheus Castanho  <msc@linux.ibm.com>

fixincludes/
	* fixinc.in: Skip machine_name fix on powerpc*-*-linux*.