[Fuchsia] Make __start_* / __stop_* symbols hidden

[phosek]

Summary

This is the intended semantics, see for more details:
https://sourceware.org/pipermail/binutils/2020-June/111685.html

[mcgrathr]

lgtm

clang/lib/Driver/ToolChains/Fuchsia.cpp

void fuchsia::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   Args.ClaimAllArgs(options::OPT_w);
 
   CmdArgs.push_back("-z");
   CmdArgs.push_back("max-page-size=4096");
 
   CmdArgs.push_back("-z");
   CmdArgs.push_back("now");
 
+  CmdArgs.push_back("-z");
+  CmdArgs.push_back("start-stop-visibility=hidden");
+
   const char *Exec = Args.MakeArgString(ToolChain.GetLinkerPath());
   if (llvm::sys::path::filename(Exec).equals_insensitive("ld.lld") ||
       llvm::sys::path::stem(Exec).equals_insensitive("ld.lld")) {
     CmdArgs.push_back("-z");
     CmdArgs.push_back("rodynamic");
     CmdArgs.push_back("-z");
     CmdArgs.push_back("separate-loadable-segments");
     CmdArgs.push_back("-z");

clang/test/Driver/fuchsia.c

 // CHECK: "-isysroot" "[[SYSROOT:[^"]+]]"
 // CHECK: "-internal-externc-isystem" "[[SYSROOT]]{{/|\\\\}}include"
 // CHECK-AARCH64: "-fsanitize=shadow-call-stack"
 // CHECK-RISCV64: "-fsanitize=shadow-call-stack"
 // CHECK-X86_64: "-fsanitize=safe-stack"
 // CHECK: "-stack-protector" "2"
 // CHECK-AARCH64: "-target-feature" "+outline-atomics"
 // CHECK-NOT: "-fcommon"
-// CHECK: {{.*}}ld.lld{{.*}}" "-z" "max-page-size=4096" "-z" "now" "-z" "rodynamic" "-z" "separate-loadable-segments" "-z" "rel" "--pack-dyn-relocs=relr"
+// CHECK: {{.*}}ld.lld{{.*}}" "-z" "max-page-size=4096" "-z" "now" "-z" "start-stop-visibility=hidden" "-z" "rodynamic" "-z" "separate-loadable-segments" "-z" "rel" "--pack-dyn-relocs=relr"
 // CHECK: "--sysroot=[[SYSROOT]]"
 // CHECK: "-pie"
 // CHECK: "--build-id"
 // CHECK: "--hash-style=gnu"
 // CHECK-AARCH64: "--execute-only" "--fix-cortex-a53-843419"
 // CHECK: "-dynamic-linker" "ld.so.1"
 // CHECK-RISCV64: "-X"
 // CHECK: Scrt1.o

clang/test/Driver/fuchsia.cpp

 // CHECK-NOT: "-fno-use-init-array"
 // CHECK: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK: "-isysroot" "[[SYSROOT:[^"]+]]"
 // CHECK-X86_64: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}c++{{/|\\\\}}v1"
 // CHECK-AARCH64: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}aarch64-unknown-fuchsia{{/|\\\\}}c++{{/|\\\\}}v1"
 // CHECK-RISCV64: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}riscv64-unknown-fuchsia{{/|\\\\}}c++{{/|\\\\}}v1"
 // CHECK: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}c++{{/|\\\\}}v1"
 // CHECK: "-internal-externc-isystem" "[[SYSROOT]]{{/|\\\\}}include"
-// CHECK: {{.*}}ld.lld{{.*}}" "-z" "now" "-z" "rodynamic" "-z" "separate-loadable-segments"
+// CHECK: {{.*}}ld.lld{{.*}}" "-z" "now" "-z" "start-stop-visibility=hidden" "-z" "rodynamic" "-z" "separate-loadable-segments" "-z" "rel" "--pack-dyn-relocs=relr"
 // CHECK: "--sysroot=[[SYSROOT]]"
 // CHECK: "-pie"
 // CHECK: "--build-id"
 // CHECK: "-dynamic-linker" "ld.so.1"
 // CHECK: Scrt1.o
 // CHECK-NOT: crti.o
 // CHECK-NOT: crtbegin.o
 // CHECK: "-L[[SYSROOT]]{{/|\\\\}}lib"