tools/nolibc: Remove .global _start from the entry point code

Building with clang yields the following error:
```
  <inline asm>:3:1: error: _start changed binding to STB_GLOBAL
  .global _start
  ^
  1 error generated.
```
Make sure only specify one between `.global _start` and `.weak _start`.
Remove `.global _start`.

Cc: llvm@lists.linux.dev
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/arch-aarch64.h

@@ -184,7 +184,6 @@ struct sys_stat_struct {
 /* startup code */
 __asm__ (".section .text\n"
     ".weak _start\n"
-    ".global _start\n"
     "_start:\n"
     "ldr x0, [sp]\n"              // argc (x0) was in the stack
     "add x1, sp, 8\n"             // argv (x1) = sp

tools/include/nolibc/arch-arm.h

@@ -177,7 +177,6 @@ struct sys_stat_struct {
 /* startup code */
 __asm__ (".section .text\n"
     ".weak _start\n"
-    ".global _start\n"
     "_start:\n"
 #if defined(__THUMBEB__) || defined(__THUMBEL__)
     /* We enter here in 32-bit mode but if some previous functions were in

tools/include/nolibc/arch-i386.h

@@ -176,7 +176,6 @@ struct sys_stat_struct {
  */
 __asm__ (".section .text\n"
     ".weak _start\n"
-    ".global _start\n"
     "_start:\n"
     "pop %eax\n"                // argc   (first arg, %eax)
     "mov %esp, %ebx\n"          // argv[] (second arg, %ebx)

tools/include/nolibc/arch-mips.h

@@ -192,7 +192,6 @@ struct sys_stat_struct {
 __asm__ (".section .text\n"
     ".weak __start\n"
     ".set nomips16\n"
-    ".global __start\n"
     ".set    noreorder\n"
     ".option pic0\n"
     ".ent __start\n"

tools/include/nolibc/arch-riscv.h

@@ -185,7 +185,6 @@ struct sys_stat_struct {
 /* startup code */
 __asm__ (".section .text\n"
     ".weak _start\n"
-    ".global _start\n"
     "_start:\n"
     ".option push\n"
     ".option norelax\n"

tools/include/nolibc/arch-x86_64.h

@@ -199,7 +199,6 @@ struct sys_stat_struct {
  */
 __asm__ (".section .text\n"
     ".weak _start\n"
-    ".global _start\n"
     "_start:\n"
     "pop %rdi\n"                // argc   (first arg, %rdi)
     "mov %rsp, %rsi\n"          // argv[] (second arg, %rsi)