[libc] Initiliaze the global pointer in riscv startup code.

[jrtc27]

1. Why store in memory on the stack only to load it again?
2. Depending on how this is compiled it could use the GOT which isn't a good idea
3. Dollars in a C identifier sound like a bad idea
4. Relaxations need to be disabled for the address calculation specifically otherwise it will be relaxed to be GP-relative, which is clearly unhelpful
5. Why is it declared as pointing to an intptr_t instead of being the usual char[]?

In short this really just needs to be done in assembly as the standard .option push; .option norelax; lla gp, __global_pointer$; .option pop.

[MaskRay]

I agree with Jessica. Assembly or inline assembly will read better.

[sivachandra]

Update as per the suggestion.

[sivachandra]

In D151539#4375402, @jrtc27 wrote:

In short this really just needs to be done in assembly as the standard .option push; .option norelax; lla gp, __global_pointer$; .option pop.

Thanks for the suggestion. I have updated now to use this exact snippet.

[mikhail.ramalho]

I've finally got around to testing this patch and it fixes the segfault I was having with clang 15 on visionfive 2.

[sivachandra]

Rebase and disable sprintf_test.hermetic as it is failing on riscv.

libc/startup/linux/riscv64/start.cpp

 using __llvm_libc::app;
 
 // TODO: Would be nice to use the aux entry structure from elf.h when available.
 struct AuxEntry {
   uint64_t type;
   uint64_t value;
 };
 
+extern "C" {
+extern intptr_t __global_pointer$;
+}
+
 __attribute__((noinline)) static void do_start() {
+  void *ptr = &__global_pointer$;
+  LIBC_INLINE_ASM("ld gp, %0\n\t" : : "m"(ptr));
   auto tid = __llvm_libc::syscall_impl(SYS_gettid);
   if (tid <= 0)
     __llvm_libc::syscall_impl(SYS_exit, 1);
   __llvm_libc::main_thread_attrib.tid = tid;
 
   // After the argv array, is a 8-byte long NULL value before the array of env
   // values. The end of the env values is marked by another 8-byte long NULL
   // value. We step over it (the "+ 1" below) to get to the env values.