tools/nolibc: add auxiliary vector retrieval for mips

In the _start block we now iterate over envp to find the auxiliary
vector after the NULL. The pointer is saved into an _auxv variable
that is marked as weak so that it's accessible from multiple units.
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/arch-mips.h

@@ -177,6 +177,7 @@ struct sys_stat_struct {
 })
 
 char **environ __attribute__((weak));
+const unsigned long *_auxv __attribute__((weak));
 
 /* startup code, note that it's called __start on MIPS */
 void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) __start(void)
@@ -196,6 +197,16 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) __start(void)
 		"lui $a3, %hi(environ)\n"     // load environ into a3 (hi)
 		"addiu $a3, %lo(environ)\n"   // load environ into a3 (lo)
 		"sw $a2,($a3)\n"              // store envp(a2) into environ
+
+		"move $t0, $a2\n"             // iterate t0 over envp, look for NULL
+		"0:"                          // do {
+		"lw $a3, ($t0)\n"             //   a3=*(t0);
+		"bne $a3, $0, 0b\n"           // } while (a3);
+		"addiu $t0, $t0, 4\n"         // delayed slot: t0+=4;
+		"lui $a3, %hi(_auxv)\n"       // load _auxv into a3 (hi)
+		"addiu $a3, %lo(_auxv)\n"     // load _auxv into a3 (lo)
+		"sw $t0, ($a3)\n"             // store t0 into _auxv
+
 		"li $t0, -8\n"
 		"and $sp, $sp, $t0\n"   // sp must be 8-byte aligned
 		"addiu $sp,$sp,-16\n"   // the callee expects to save a0..a3 there!