std.Target: remove Abi.cygnus

There is approximately zero chance of the Zig team ever spending any effort on
supporting Cygwin; the MSVC and MinGW-w64 ABIs are superior in every way that
matters, and not least because they lead to binaries that just run natively on
Windows without needing a POSIX emulation environment installed.

src/codegen/llvm.zig

@@ -295,7 +295,6 @@ pub fn targetTriple(allocator: Allocator, target: *const std.Target) ![]const u8
         .muslx32 => "muslx32",
         .msvc => "msvc",
         .itanium => "itanium",
-        .cygnus => "cygnus",
         .simulator => "simulator",
         .macabi => "macabi",
         .ohos, .ohoseabi => "ohos",
@@ -420,7 +419,6 @@ pub fn dataLayout(target: *const std.Target) []const u8 {
         .sparc64 => "E-m:e-i64:64-i128:128-n32:64-S128",
         .s390x => "E-m:e-i1:8:16-i8:8:16-i64:64-f128:64-v128:64-a:8:16-n32:64",
         .x86 => if (target.os.tag == .windows or target.os.tag == .uefi) switch (target.abi) {
-            .cygnus => "e-m:x-p:32:32-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:32-n8:16:32-a:0:32-S32",
             .gnu => if (target.ofmt == .coff)
                 "e-m:x-p:32:32-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:32-n8:16:32-a:0:32-S32"
             else

src/target.zig

@@ -63,7 +63,7 @@ pub fn supports_fpic(target: *const std.Target) bool {
     return switch (target.os.tag) {
         .windows,
         .uefi,
-        => target.abi == .gnu or target.abi == .cygnus,
+        => target.abi == .gnu,
         else => true,
     };
 }
@@ -93,7 +93,6 @@ pub fn useEmulatedTls(target: *const std.Target) bool {
     if (target.abi.isOpenHarmony()) return true;
     return switch (target.os.tag) {
         .openbsd => true,
-        .windows => target.abi == .cygnus,
         else => false,
     };
 }