move call to dynamic linker stage-3 into stage-2 function

this move eliminates a duplicate "by-hand" symbol lookup loop from the
stage-1 code and replaces it with a call to find_sym, which can be
used once we're in stage 2. it reduces the size of the stage 1 code,
which is helpful because stage 1 will become the crt start file for
static-PIE executables, and it will allow stage 3 to access stage 2's
automatic storage, which will be important in an upcoming commit.

src/internal/dynlink.h

@@ -51,7 +51,7 @@ enum {
 #define AUX_CNT 32
 #define DYN_CNT 32
 
-typedef void (*stage2_func)(unsigned char *);
+typedef void (*stage2_func)(unsigned char *, size_t *);
 typedef _Noreturn void (*stage3_func)(size_t *);
 
 #endif

src/ldso/dlstart.c

@@ -84,16 +84,7 @@ void _dlstart_c(size_t *sp, size_t *dynv)
 		 && s[3]=='l' && s[4]=='s' && s[5]=='2' && !s[6])
 			break;
 	}
-	((stage2_func)(base + syms[i].st_value))(base);
-
-	/* Call dynamic linker stage-3, __dls3 */
-	for (i=0; ;i++) {
-		const char *s = strings + syms[i].st_name;
-		if (s[0]=='_' && s[1]=='_' && s[2]=='d'
-		 && s[3]=='l' && s[4]=='s' && s[5]=='3' && !s[6])
-			break;
-	}
-	((stage3_func)(base + syms[i].st_value))(sp);
+	((stage2_func)(base + syms[i].st_value))(base, sp);
 }
 
 #endif

src/ldso/dynlink.c

@@ -1116,7 +1116,7 @@ static void update_tls_size()
  * linker itself, but some of the relocations performed may need to be
  * replaced later due to copy relocations in the main program. */
 
-void __dls2(unsigned char *base)
+void __dls2(unsigned char *base, size_t *sp)
 {
 	Ehdr *ehdr = (void *)base;
 	ldso.base = base;
@@ -1134,6 +1134,12 @@ void __dls2(unsigned char *base)
 
 	ldso.relocated = 0;
 	ldso.rel_update_got = 1;
+
+	/* Call dynamic linker stage-3, __dls3, looking it up
+	 * symbolically as a barrier against moving the address
+	 * load across the above relocation processing. */
+	struct symdef dls3_def = find_sym(&ldso, "__dls3", 0);
+	((stage3_func)(ldso.base+dls3_def.sym->st_value))(sp);
 }
 
 /* Stage 3 of the dynamic linker is called with the dynamic linker/libc