std.elf: Fix read functions for 32-bit targets
The buffer index was declared as `u64`, which overflows `usize` on a
32-bit target.
The following example program failed to compile for 32-bit targets:
```zig
const std = @import("std");
pub fn main() !void {
const alloc = std.testing.allocator;
const file = std.io.getStdIn();
_ = try std.elf.readAllHeaders(alloc, file);
}
```
```
lib/zig/std/elf.zig:543:36: error: expected type 'usize', found 'u64'
const len = file.pread(buf[i .. buf.len - i], offset + i) catch |err| switch (err) {
^
lib/zig/std/elf.zig:543:36: note: unsigned 32-bit int cannot represent all possible unsigned 64-bit values
const len = file.pread(buf[i .. buf.len - i], offset + i) catch |err| switch (err) {
^
lib/zig/std/elf.zig:543:35: note: referenced here
const len = file.pread(buf[i .. buf.len - i], offset + i) catch |err| switch (err) {
^
lib/zig/std/elf.zig:348:5: note: referenced here
try preadNoEof(file, &hdr_buf, 0);
^
lib/zig/std/elf.zig:392:19: note: referenced here
.header = try readHeader(file),
```