[PGO][AIX] Disable multi-process continuous mode test in 32-bit

In PGO continuous mode, we mmap the profile file into shared memory, which
allows multiple processes to be updating the same memory.

The -fprofile-update=atomic option forces the counter increments to be atomic,
but the counter size is always 64-bit (in -m32 and -m64), so in 32-bit mode the
atomic operations are function calls to libatomic.a and these function calls use
locks.

The lock based libatomic.a functions are per-process, so two processes will race
on the same shared memory because each will acquire their own lock.

Author: Wael Yehia

compiler-rt/test/profile/ContinuousSyncMode/online-merging.c

@@ -1,4 +1,5 @@
 // REQUIRES: continuous-mode
+// UNSUPPORTED: powerpc-{{.*}}
 
 // Test the online merging mode (%m) along with continuous mode (%c).
 //