elf: add basic TLS segment handling

[kubkon]

It's not exactly efficient as .tbss is actually never mapped at runtime unlike .bss. For that reason, typically linker overlaps .tbss with a succeeding alloc section, usually .data, in virtual memory, since we only care about the continuity of .tdata into .tbss. When the relocations are resolved, they are lowered into positive and negative offsets wrt to the TLS segment. I think given this knowledge there exists the potential for improving current implementation by quite a bit.

In other news, I have now also added a new test target to the test matrix:

+        .{
+            .target = .{
+                .cpu_arch = .x86_64,
+                .os_tag = .linux,
+                .abi = .musl,
+            },
+            .link_libc = true,
+            .use_lld = false,
+        },