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, 15 insertions, 0 deletions

diff --git a/lib/c/common.zig b/lib/c/common.zig
new file mode 100644
index 0000000000..e9536f63b4
--- /dev/null
+++ b/lib/c/common.zig
@@ -0,0 +1,15 @@
+const builtin = @import("builtin");
+const std = @import("std");
+
+pub const linkage: std.builtin.GlobalLinkage = if (builtin.is_test)
+    .internal
+else
+    .strong;
+
+/// Determines the symbol's visibility to other objects.
+/// For WebAssembly this allows the symbol to be resolved to other modules, but will not
+/// export it to the host runtime.
+pub const visibility: std.builtin.SymbolVisibility = if (builtin.cpu.arch.isWasm() and linkage != .internal)
+    .hidden
+else
+    .default;