Introduce libzigc for libc function implementations in Zig.

This lays the groundwork for #2879. This library will be built and linked when a
static libc is going to be linked into the compilation. Currently, that means
musl, wasi-libc, and MinGW-w64. As a demonstration, this commit removes the musl
C code for a few string functions and implements them in libzigc. This means
that those libzigc functions are now load-bearing for musl and wasi-libc.

Note that if a function has an implementation in compiler-rt already, libzigc
should not implement it. Instead, as we recently did for memcpy/memmove, we
should delete the libc copy and rely on the compiler-rt implementation.

I repurposed the existing "universal libc" code to do this. That code hadn't
seen development beyond basic string functions in years, and was only usable-ish
on freestanding. I think that if we want to seriously pursue the idea of Zig
providing a freestanding libc, we should do so only after defining clear goals
(and non-goals) for it. See also #22240 for a similar case.

1 files changed, 4 insertions, 10 deletions

diff --git a/src/link/Elf.zig b/src/link/Elf.zig
index 53f88101b1..00bd940500 100644
--- a/src/link/Elf.zig
+++ b/src/link/Elf.zig
@@ -1984,16 +1984,6 @@ fn linkWithLLD(self: *Elf, arena: Allocator, tid: Zcu.PerThread.Id, prog_node: s
             try argv.append(try p.toString(arena));
         }
 
-        // libc
-        if (is_exe_or_dyn_lib and
-            !comp.skip_linker_dependencies and
-            !comp.config.link_libc)
-        {
-            if (comp.libc_static_lib) |lib| {
-                try argv.append(try lib.full_object_path.toString(arena));
-            }
-        }
-
         // Shared libraries.
         if (is_exe_or_dyn_lib) {
             // Worst-case, we need an --as-needed argument for every lib, as well
@@ -2071,6 +2061,10 @@ fn linkWithLLD(self: *Elf, arena: Allocator, tid: Zcu.PerThread.Id, prog_node: s
                 } else {
                     diags.flags.missing_libc = true;
                 }
+
+                if (comp.zigc_static_lib) |zigc| {
+                    try argv.append(try zigc.full_object_path.toString(arena));
+                }
             }
         }