[Flang-RT] Make implicit PPC intrinsic uses explicit (#206493)

The PPC-specific builtin moduls have implicit dependencies insert by
flang. For CMake to correctly setup the build order, they must be made
explicit.

This replicates the implicit module uses from

12b20b3673/flang/lib/Semantics/semantics.cpp (L680-L693)
in the modules themselves. The PPC-specific modules compiled by Flang-RT
are listed here:

12b20b3673/flang-rt/lib/runtime/CMakeLists.txt (L213-L219)

This should fix the intermittend build error mentioned in
https://github.com/llvm/llvm-project/pull/205634#issuecomment-4829653483.
Unfortunately I do not have a PowerPC (cross-)compilation build
environment ready to test, nor does the Premerge-CI build it.

flang-rt/lib/runtime/__ppc_intrinsics.f90

@@ -7,6 +7,7 @@
 !===------------------------------------------------------------------------===!
 
 module __ppc_intrinsics
+  use __ppc_types ! implicit dependencies, made explicit for CMake
 
   private
 

flang-rt/lib/runtime/__ppc_types.f90

@@ -7,6 +7,7 @@
 !===------------------------------------------------------------------------===!
 
 module __ppc_types
+  use __fortran_builtins ! implicit dependency, made explicit for CMake
   implicit none
 
   ! Set PRIVATE by default to explicitly only export what is meant

flang-rt/lib/runtime/mma.f90

@@ -7,6 +7,8 @@
 !===------------------------------------------------------------------------===!
 
 module mma
+  use __ppc_types ! implicit dependencies, made explicit for CMake
+
   implicit none
   private