Fix slight deviation from spec in handling Elf*_Rela relative relocations

`Elf*_Rela` relocations store their argument in `r_addend`, including for `R_*_RELATIVE` relocations.  Unlike `Elf*_Rel` relocations, they are not applied as a delta to the destination virtual address.  Instead, they are computed from `base_address + r_addend` directly.

Author: InBetweenNames

lib/std/os/linux/start_pie.zig

@@ -113,7 +113,7 @@ pub fn relocate(phdrs: []elf.Phdr) void {
         const rela = std.mem.bytesAsSlice(elf.Rela, @as([*]u8, @ptrFromInt(rela_addr))[0..rela_size]);
         for (rela) |r| {
             if (r.r_type() != R_RELATIVE) continue;
-            @as(*usize, @ptrFromInt(base_addr + r.r_offset)).* += base_addr + @as(usize, @bitCast(r.r_addend));
+            @as(*usize, @ptrFromInt(base_addr + r.r_offset)).* = base_addr + @as(usize, @bitCast(r.r_addend));
         }
     }
 }