Omit frame pointer also for main in ppc ldst_multiple test.

Omit frame pointer also for main in ppc ldst_multiple test.

Other functions already explicitly omitted the frame pointer. Also
do that for main to prevent gcc 4.8.2 complaining:

 ldst_multiple.c: In function ‘main’:
 ldst_multiple.c:180:5: error: frame pointer required, but reserved
  int main(void)
      ^
 ldst_multiple.c:31:18: note: for ‘r31’
  register HWord_t r31 asm("r31");

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14173