Support LP64 platforms such as Darwin where int64_t is long long. (#7596)

Add `Core.CppCompat.[U]Long64` to represent a 64-bit long that is not
`i64`. Treat it as being "just slightly smaller than" `i64`, like we
treat `Core.CppCompat.LongLong64` as being "just slightly larger than"
`i64`, so that we get implicit conversions `Cpp.long` -> `i64` ->
`Cpp.long_long` on all targets.

This follows the direction of proposal #5448, and seems like the obvious
extension of the `[U]Long32` and `[U]LongLong64` types added in #6275
for targets of this "shape".

Assisted-by: Gemini via Antigravity

toolchain/check/testdata/interop/cpp/function/import/arithmetic_types_direct.carbon

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // INCLUDE-FILE: toolchain/testing/testdata/min_prelude/primitives.carbon
+// EXTRA-ARGS: --target=x86_64-linux-gnu
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run: