[Driver] Use --warn-shared-textrel for Android.

[danalbert]

Summary

Android does not allow shared text relocations. Enable the linker
warning to detect them by default.

lib/Driver/ToolChains/Gnu.cpp

void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   // Most Android ARM64 targets should enable the linker fix for erratum
   // 843419. Only non-Cortex-A53 devices are allowed to skip this flag.
   if (Arch == llvm::Triple::aarch64 && isAndroid) {
     std::string CPU = getCPUName(Args, Triple);
     if (CPU.empty() || CPU == "generic" || CPU == "cortex-a53")
       CmdArgs.push_back("--fix-cortex-a53-843419");
   }
 
+  // Android does not allow shared text relocations. Emit a warning if the
+  // user's code contains any.
+  if (isAndroid)
+      CmdArgs.push_back("--warn-shared-textrel");
+
   for (const auto &Opt : ToolChain.ExtraOpts)
     CmdArgs.push_back(Opt.c_str());
 
   CmdArgs.push_back("--eh-frame-hdr");
 
   if (const char *LDMOption = getLDMOption(ToolChain.getTriple(), Args)) {
     CmdArgs.push_back("-m");
     CmdArgs.push_back(LDMOption);

test/Driver/linux-ld.c

 // RUN: %clang %s -### -o %t.o 2>&1 \
 // RUN:     --target=armv7-linux-android21 \
 // RUN:   | FileCheck --check-prefix=CHECK-ANDROID-NOEXECSTACK %s
 // CHECK-ANDROID-NOEXECSTACK: "{{.*}}ld{{(.exe)?}}"
 // CHECK-ANDROID-NOEXECSTACK: "-z" "noexecstack"
 // CHECK-ANDROID-NOEXECSTACK-NOT: "-z" "execstack"
 // CHECK-ANDROID-NOEXECSTACK-NOT: "-z,execstack"
 // CHECK-ANDROID-NOEXECSTACK-NOT: "-zexecstack"
-//
+
++// RUN: %clang %s -### -o %t.o 2>&1 \
++// RUN:     --target=armv7-linux-android21 \
++// RUN:   | FileCheck --check-prefix=CHECK-ANDROID-WARN-SHARED-TEXTREL %s
++// CHECK-ANDROID-WARN-SHARED-TEXTREL: "{{.*}}ld{{(.exe)?}}"
++// CHECK-ANDROID-WARN-SHARED-TEXTREL: "--warn-shared-textrel"
+
 // RUN: %clang %s -### -o %t.o 2>&1 --target=mips64-linux-gnuabin32 \
 // RUN:   | FileCheck --check-prefix=CHECK-MIPS64EL-GNUABIN32 %s
 // CHECK-MIPS64EL-GNUABIN32: "{{.*}}ld{{(.exe)?}}"
 // CHECK-MIPS64EL-GNUABIN32: "-m" "elf32btsmipn32"
 // CHECK-MIPS64EL-GNUABIN32: "-dynamic-linker" "{{.*}}/lib{{(32)?}}/ld.so.1"
 // CHECK-MIPS64EL-GNUABIN32-NOT: "--hash-style={{gnu|both}}"
 //
 // RUN: %clang %s -### -o %t.o 2>&1 --target=mips64-linux-gnuabi64 \