[msan] Change #ifdef SANITIZER_PPC to #if (#94009)

https://github.com/llvm/llvm-project/commit/0e96eebc7f681a7ce41f35909e609c7c61a11455
accidentally turned the prior patch
(https://github.com/llvm/llvm-project/commit/57a507930b50c445140feb68bffe1c21af53319e)
into a no-op because this macro is always defined (as either 1 or 0).
This patch changes it to correctly use #if.