std.os: add APIs to determine whether Zig std requires libc for a target

This is distinct from the question of whether the target formally considers libc
to be the only stable syscall interface; for example, FreeBSD and NetBSD have
stable syscalls, but we don't yet have syscall layers for them in std.os.

lib/std/os.zig

@@ -1,4 +1,5 @@
 const builtin = @import("builtin");
+const std = @import("std.zig");
 const native_os = builtin.os.tag;
 
 pub const linux = @import("os/linux.zig");
@@ -8,6 +9,46 @@ pub const wasi = @import("os/wasi.zig");
 pub const emscripten = @import("os/emscripten.zig");
 pub const windows = @import("os/windows.zig");
 
+/// Returns whether the Zig standard library requires libc in order to interface
+/// with the operating system on the given target.
+pub fn targetRequiresLibC(target: *const std.Target) bool {
+    if (target.requiresLibC()) return true;
+    return switch (target.os.tag) {
+        .linux => switch (target.cpu.arch) {
+            // https://codeberg.org/ziglang/zig/issues/30940
+            .alpha,
+            // https://codeberg.org/ziglang/zig/issues/30942
+            .csky,
+            // https://codeberg.org/ziglang/zig/issues/30943
+            .hppa,
+            .hppa64,
+            // https://codeberg.org/ziglang/zig/issues/30944
+            .microblaze,
+            .microblazeel,
+            // https://codeberg.org/ziglang/zig/issues/30946
+            .sh,
+            .sheb,
+            // https://codeberg.org/ziglang/zig/issues/30945
+            .sparc,
+            // https://codeberg.org/ziglang/zig/issues/30947
+            .xtensa,
+            .xtensaeb,
+            => true,
+            else => false,
+        },
+        .freebsd => true, // https://codeberg.org/ziglang/zig/issues/30981
+        .netbsd => true, // https://codeberg.org/ziglang/zig/issues/30980
+        .openbsd => true, // https://codeberg.org/ziglang/zig/issues/30982
+        else => false,
+    };
+}
+
+/// Returns whether the Zig standard library requires libc in order to interface
+/// with the operating system on the current target.
+pub fn requiresLibC() bool {
+    return targetRequiresLibC(&builtin.target);
+}
+
 test {
     _ = linux;
     if (native_os == .uefi) _ = uefi;

src/Compilation/Config.zig

@@ -234,19 +234,10 @@ pub fn resolve(options: Options) ResolveError!Config {
             break :b true;
         }
         if (options.link_libc) |x| break :b x;
-        switch (target.os.tag) {
-            // These targets don't require libc, but we don't yet have a syscall layer for them,
-            // so we default to linking libc for now.
-            .freebsd,
-            .netbsd,
-            .openbsd,
-            => break :b true,
-            else => {},
-        }
         if (options.ensure_libc_on_non_freestanding and target.os.tag != .freestanding)
             break :b true;
 
-        break :b target.requiresLibC();
+        break :b std.os.targetRequiresLibC(target);
     };
 
     const link_mode = b: {

test/src/StackTrace.zig

@@ -58,10 +58,7 @@ fn addCaseTarget(
         .fuchsia => false,
         else => true,
     };
-    const both_libc = switch (target.result.os.tag) {
-        .freebsd, .netbsd, .openbsd => false,
-        else => !target.result.requiresLibC(),
-    };
+    const both_libc = !std.os.targetRequiresLibC(&target.result);
 
     // See `std.debug.StackIterator.fp_usability` logic.
     const fp_usability: enum { useless, unsafe, safe, ideal } = switch (target.result.cpu.arch) {