[libc] Declare __dso_handle in the integration test instead of startup.

[sivachandra]

Summary

Fixes #61355. The __dso_handle decl was introduced incorrectly into the startup
objects during the integration test cleanup which moved the integration tests
away from using an artificial sysroot to using -nostdlib. Having it in the
startup creates the duplicate symbol error when one does not use -nostdlib.
Since this is an integration test only problem, it is meaningful to keep it in
the integration test anyway.

[sivachandra]

Sorry, I did not intend to push this without review. Because of a mix of branches on my local checkout, I ended up pushing it accidentally.

libc/startup/linux/aarch64/start.cpp

 #include <linux/auxvec.h>
 #include <linux/elf.h>
 #include <stdint.h>
 #include <sys/mman.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
 extern "C" int main(int, char **, char **);
-// The BFD linker requires a reference to __dso_handle to trigger creating
-// a symbol for it when -nostdlib is used..
-extern "C" void *__dso_handle = nullptr;
 
 // Source documentation:
 // https://github.com/ARM-software/abi-aa/tree/main/sysvabi64
 
 namespace __llvm_libc {
 
 #ifdef SYS_mmap2
 static constexpr long MMAP_SYSCALL_NUMBER = SYS_mmap2;

libc/startup/linux/riscv64/start.cpp

 #include <linux/auxvec.h>
 #include <linux/elf.h>
 #include <stdint.h>
 #include <sys/mman.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
 extern "C" int main(int, char **, char **);
-// The BFD linker requires a reference to __dso_handle to trigger creating
-// a symbol for it when -nostdlib is used..
-extern "C" void *__dso_handle = nullptr;
 
 namespace __llvm_libc {
 
 #ifdef SYS_mmap2
 static constexpr long MMAP_SYSCALL_NUMBER = SYS_mmap2;
 #elif SYS_mmap
 static constexpr long MMAP_SYSCALL_NUMBER = SYS_mmap;
 #else

libc/startup/linux/x86_64/start.cpp

 #include <linux/auxvec.h>
 #include <linux/elf.h>
 #include <stdint.h>
 #include <sys/mman.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
 extern "C" int main(int, char **, char **);
-// The BFD linker requires a reference to __dso_handle to trigger creating
-// a symbol for it when -nostdlib is used..
-extern "C" void *__dso_handle = nullptr;
 
 namespace __llvm_libc {
 
 #ifdef SYS_mmap2
 static constexpr long mmapSyscallNumber = SYS_mmap2;
 #elif SYS_mmap
 static constexpr long mmapSyscallNumber = SYS_mmap;
 #else

libc/test/IntegrationTest/test.cpp

 
 void free(void *) {}
 
 void *realloc(void *ptr, size_t s) {
   free(ptr);
   return malloc(s);
 }
 
+// Integration tests are linked with -nostdlib. BFD linker expects
+// __dso_handle when -nostdlib is used.
+void *__dso_handle = nullptr;
 } // extern "C"