[ARM,PowerPC] shouldForceRelocation: check MCValue::Specifier

Follow-up to 38c3ad3

Removes reliance on a MCAssembler::evaluateFixup hack
`if (Target.SymSpecifier || SA.isUndefined()) {` (previosuly
`if (A->getKind() != MCSymbolRefExpr::VK_None || SA.isUndefined()) {`)

Target.SymSpecifier will soon go away when MCValue replaces
MCSymbolRefExpr members with MCSymbol and removes the temporary
`SymSpecifier` workaround.

Author: MaskRay

llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp

@@ -993,7 +993,7 @@ bool ARMAsmBackend::shouldForceRelocation(const MCAssembler &Asm,
               FixupKind == ARM::fixup_arm_uncondbl ||
               FixupKind == ARM::fixup_arm_condbl))
     return true;
-  return false;
+  return Target.getSpecifier();
 }
 
 /// getFixupKindNumBytes - The number of bytes the fixup may change.

llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp

@@ -159,6 +159,10 @@ class PPCAsmBackend : public MCAsmBackend {
   bool shouldForceRelocation(const MCAssembler &Asm, const MCFixup &Fixup,
                              const MCValue &Target,
                              const MCSubtargetInfo *STI) override {
+    // If there is a @ specifier, unless it is optimized out (e.g. constant @l),
+    // force a relocation.
+    if (Target.getSpecifier())
+      return true;
     MCFixupKind Kind = Fixup.getKind();
     switch ((unsigned)Kind) {
     default: