Some compilers such as Go reference the end of a section (addr + size)
which cannot be contained in any non-zero atom (since then this atom
would exceed section boundaries). In order to facilitate this behaviour,
we create a dummy zero-sized atom at section end (addr + size).
By organizing linker diagnostics into this struct, it becomes possible
to share more code between linker backends, and more importantly it
becomes possible to pass only the Diag struct to some functions, rather
than passing the entire linker state object in. This makes data
dependencies more obvious, making it easier to rearrange code and to
multithread.
Also fix MachO code abusing an atomic variable. Not only was it using
the wrong atomic operation, it is unnecessary additional state since
the state is already being protected by a mutex.
In order to reduce the logic that happens in flush() we need to see
which data is being accessed by all this logic, so we can see which
operations depend on each other.
`check_pie_supported` only uses the `OUTPUT_VARIABLE` to to signify errors
if PIE is actually supported is signaled by `CMAKE_<lang>_LINK_PIE_SUPPORTED`.
Checking if `OUTPUT_VARIABLE` is empty is not enough either since the check
is bypassed if its results are cached but the output variable is not cached.
When errors occurred during flush(), incremental cache mode was still
writing a successful cache manifest, making subsequent compilations fail
because they would get a cache hit only to find invalid data.
Embrace the Path abstraction, doing more operations based on directory
handles rather than absolute file paths. Most of the diff noise here
comes from this one.
Fix sorting of crtbegin/crtend atoms. Previously it would look at all
path components for those strings.
Make the C runtime path detection partially a pure function, and move
some logic to glibc.zig where it belongs.
The initAtoms function now only uses the `elf_file` parameter for
reporting linker error messages, making it easier to see that the
function has no data dependencies other than the Object struct itself,
making it easier to parallelize or otherwise move that logic around.
Also removed an indirect call via `addExtra` since we already know the
atom's file is the current Object instance. All calls to `Atom.addExtra`
should be audited for similar reasons.
Also removed unjustified use of `inline fn`.
Special symbols include explictly force undefined symbols passed via -u
flag, missing entry point symbol, missing 'dyld_stub_binder' symbol, or
missing '_objc_msgsend' symbol.
