[ld.lld] --gc-sections only when output is executable

When building object files, `zig cc` will instruct lld to remove unused
sections via `--gc-sections`. This is problematic cgo builds that don't
explicitly use C code. Briefly, go builds cgo executables as follows*:

1. build `_cgo_.o`, which links (on linux_amd64 systems) to
   `/usr/local/go/src/runtime/race/race_linux_amd64.syso`.
2. That `.syso` contains references to symbols from libc.

If the user program uses at least one libc symbol, it will link
correctly. However, if Go is building a cgo executable, but without C
code, the sections from `.syso` file will be garbage-collected, leaving
a `_cgo_.o` without any references to libc, causing the final linking
step to not link libc.

Until now, this could be worked around by `-linkmode external` flag to
`go build`. This causes Go to link the final executable using the
external linker (which implicitly links libc). However, that flag brings
in a whole different world of worms.

I assume the `gc_sections` is an optimization; I tried to re-add
`--gc-sections` to the final executable, but that didn't go well. I know
removing such an optimization may be contentious, so let's start the
discussion here. Quoting @andrewrk in [1] (it was about `--as-needed`,
but the point remains the same):

> The C source code for the temporary executable needs to have dummy
> calls to getuid, pthread_self, sleep, and every other libc function
> that cgo/race code wants to call.

I agree this is how it *should* work. However, while we could fix it for
go, I don't know how many other systems rely on that, and we'll never
know we've fixed the last one. The point is, GCC/Clang does not optimize
sections by default, and downstream tools rely on that. If we want to
consider `zig cc` a drop-in clang replacement (except for
`-fsanitize=undefined`, which I tend to agree with), then it should not
be optimizing the intermediate object files. Or at least have a very
prominent fine-print that this is happening, with ways to work around
it.

Fixes #11398
Fixes golang/go#44695
Fixes golang/go#52690

[*]: Empirically observed with `CGO_ENABLED=1 go test -race -x -v`

[1]: golang/go#52690 (comment)

Author: motiejus

src/link/Elf.zig

@@ -1264,7 +1264,7 @@ fn linkWithLLD(self: *Elf, comp: *Compilation, prog_node: *std.Progress.Node) !v
     const have_dynamic_linker = self.base.options.link_libc and
         self.base.options.link_mode == .Dynamic and is_exe_or_dyn_lib;
     const target = self.base.options.target;
-    const gc_sections = self.base.options.gc_sections orelse !is_obj;
+    const gc_sections = self.base.options.gc_sections orelse false;
     const stack_size = self.base.options.stack_size_override orelse 16777216;
     const allow_shlib_undefined = self.base.options.allow_shlib_undefined orelse !self.base.options.is_native_os;
     const compiler_rt_path: ?[]const u8 = blk: {