implement thread-local storage when not using LLVM or LLD

[andrewrk]

const std = @import("std");

test "thread local variables" {
    var value1: i32 = 0;
    var value2: i32 = 0;

    const thread1 = try std.Thread.spawn(.{}, run, .{&value1});
    const thread2 = try std.Thread.spawn(.{}, run, .{&value2});

    thread1.join();
    thread2.join();

    try std.testing.expectEqual(@as(i32, 1235), value1);
    try std.testing.expectEqual(@as(i32, 1235), value2);
    try std.testing.expectEqual(@as(i32, 1234), x);
}

threadlocal var x: i32 = 1234;

fn run(value: *i32) void {
    x += 1;
    value.* = x;
}

$ stage3/bin/zig test test.zig  -fllvm -flld
All 1 tests passed.

$ stage3/bin/zig test test.zig  -fno-llvm -fno-lld -fno-single-threaded
Test [1/1] test.thread local variables... expected 1235, found 1236
Test [1/1] test.thread local variables... FAIL (TestExpectedEqual)
0 passed; 0 skipped; 1 failed.
error: the following test command failed with exit code 1:
/home/andy/Downloads/zig/zig-cache/o/a01721253d2ad18ca1ea314f4ec3169e/test

[andrewrk]

zig/src/Compilation.zig

Lines 1124 to 1129 in 98dc28b

	const single_threaded = options.single_threaded orelse must_single_thread or
	// x86_64 codegen doesn't support TLV for most object formats
	(!use_llvm and options.target.cpu.arch == .x86_64 and options.target.ofmt != .macho);
	if (must_single_thread and !single_threaded) {
	return error.TargetRequiresSingleThreaded;
	}

[kubkon]

Implemented by #17978.

$ stage3/bin/zig test test.zig -fno-llvm -fno-lld -fno-single-threaded
All 1 tests passed.
$ stage3/bin/zig test test.zig -fno-llvm -fno-lld -fno-single-threaded -fPIC
All 1 tests passed.