Clang/MIPS: Use -mnan value for -mabs if not specified

On most hardware, FCSR.ABS2008 is set the value same with FCSR.NAN2008.
Let's use this behaivor by default.

With this commit, `clang -target mips -mnan=2008 -c fabs.c` will
imply `-mabs=2008`.

And of course, `clang -mnan=2008 -mabs=legacy` can continue workable
like previous.

Author: YunQiang Su

clang/lib/Driver/ToolChains/Arch/Mips.cpp

@@ -221,6 +221,7 @@ void mips::getMIPSTargetFeatures(const Driver &D, const llvm::Triple &Triple,
   bool IsN64 = ABIName == "64";
   bool IsPIC = false;
   bool NonPIC = false;
+  bool HasNaN2008Opt = false;
 
   Arg *LastPICArg = Args.getLastArg(options::OPT_fPIC, options::OPT_fno_PIC,
                                     options::OPT_fpic, options::OPT_fno_pic,
@@ -285,9 +286,10 @@ void mips::getMIPSTargetFeatures(const Driver &D, const llvm::Triple &Triple,
   if (Arg *A = Args.getLastArg(options::OPT_mnan_EQ)) {
     StringRef Val = StringRef(A->getValue());
     if (Val == "2008") {
-      if (mips::getIEEE754Standard(CPUName) & mips::Std2008)
+      if (mips::getIEEE754Standard(CPUName) & mips::Std2008) {
         Features.push_back("+nan2008");
-      else {
+        HasNaN2008Opt = true;
+      } else {
         Features.push_back("-nan2008");
         D.Diag(diag::warn_target_unsupported_nan2008) << CPUName;
       }
@@ -323,6 +325,8 @@ void mips::getMIPSTargetFeatures(const Driver &D, const llvm::Triple &Triple,
       D.Diag(diag::err_drv_unsupported_option_argument)
           << A->getSpelling() << Val;
     }
+  } else if (HasNaN2008Opt) {
+    Features.push_back("+abs2008");
   }
 
   AddTargetFeature(Args, Features, options::OPT_msingle_float,

clang/test/Driver/mips-features.c

@@ -213,7 +213,13 @@
 // RUN: %clang -target mips-linux-gnu -march=mips32r3 -### -c %s \
 // RUN:     -mnan=legacy -mnan=2008 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NAN2008 %s
-// CHECK-NAN2008: "-target-feature" "+nan2008"
+// CHECK-NAN2008: "-target-feature" "+nan2008" "-target-feature" "+abs2008"
+//
+// -mnan=2008 -mabs=legacy
+// RUN: %clang -target mips-linux-gnu -march=mips32r3 -### -c %s \
+// RUN:     -mabs=legacy -mnan=2008 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-ABSLEGACYNAN2008 %s
+// CHECK-ABSLEGACYNAN2008: "-target-feature" "+nan2008" "-target-feature" "-abs2008"
 //
 // -mnan=legacy
 // RUN: %clang -target mips-linux-gnu -march=mips32r3 -### -c %s \