[PowerPC] Fix powerpcspe subtarget enablement in llvm backend

Summary:
As currently written, -target powerpcspe will enable SPE regardless of
disabling the feature later on in the command line.  Instead, change
this to just set a default CPU to 'e500' instead of a generic CPU.

As part of this, add FeatureSPE to the e500 definition.

Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D72673

llvm/lib/Target/PowerPC/PPC.td

@@ -442,7 +442,7 @@ def : ProcessorModel<"g5", G5Model,
 def : ProcessorModel<"e500", PPCE500Model,
                   [DirectiveE500,
                    FeatureICBT, FeatureBookE,
-                   FeatureISEL, FeatureMFTB]>;
+                   FeatureISEL, FeatureMFTB, FeatureSPE]>;
 def : ProcessorModel<"e500mc", PPCE500mcModel,
                   [DirectiveE500mc,
                    FeatureSTFIWX, FeatureICBT, FeatureBookE,

llvm/lib/Target/PowerPC/PPCSubtarget.cpp

@@ -127,6 +127,8 @@ void PPCSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
     // If cross-compiling with -march=ppc64le without -mcpu
     if (TargetTriple.getArch() == Triple::ppc64le)
       CPUName = "ppc64le";
+    else if (TargetTriple.getSubArch() == Triple::PPCSubArch_spe)
+      CPUName = "e500";
     else
       CPUName = "generic";
   }
@@ -151,9 +153,6 @@ void PPCSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
       TargetTriple.isMusl())
     SecurePlt = true;
 
-  if (TargetTriple.getSubArch() == Triple::PPCSubArch_spe)
-    HasSPE = true;
-
   if (HasSPE && IsPPC64)
     report_fatal_error( "SPE is only supported for 32-bit targets.\n", false);
   if (HasSPE && (HasAltivec || HasQPX || HasVSX || HasFPU))