std.Target: Remove the gnuf64 ABI specifier.

This was used for LoongArch64, where:

* `gnuf64` -> `ilp32d` / `lp64d` (full hard float)
* `gnuf32` -> `ilp32f` / `lp64f` (hard float for `f32` only)
* `gnusf` -> `ilp32` / `lp64` (soft float)

But Loongson eventually settled on just `gnu` for the first case since that's
what most people will actually be targeting outside embedded scenarios. The
`gnuf32` and `gnusf` specifiers remain in use.

lib/compiler/aro/aro/target.zig

@@ -737,7 +737,6 @@ pub fn toLLVMTriple(target: std.Target, buf: []u8) []const u8 {
         .gnueabi => "gnueabi",
         .gnueabihf => "gnueabihf",
         .gnuf32 => "gnuf32",
-        .gnuf64 => "gnuf64",
         .gnusf => "gnusf",
         .gnux32 => "gnux32",
         .gnuilp32 => "gnuilp32",

lib/std/Target.zig

@@ -628,7 +628,6 @@ pub const Abi = enum {
     gnueabi,
     gnueabihf,
     gnuf32,
-    gnuf64,
     gnusf,
     gnux32,
     gnuilp32,

lib/std/zig/LibCDirs.zig

@@ -232,7 +232,6 @@ fn libCGenericName(target: std.Target) [:0]const u8 {
         .gnueabi,
         .gnueabihf,
         .gnuf32,
-        .gnuf64,
         .gnusf,
         .gnux32,
         .gnuilp32,

src/codegen/llvm.zig

@@ -177,7 +177,6 @@ pub fn targetTriple(allocator: Allocator, target: std.Target) ![]const u8 {
         .gnueabi => "gnueabi",
         .gnueabihf => "gnueabihf",
         .gnuf32 => "gnuf32",
-        .gnuf64 => "gnuf64",
         .gnusf => "gnusf",
         .gnux32 => "gnux32",
         .gnuilp32 => "gnuilp32",