musl: Clarify reason for CPU_SETSIZE value

This value was set to 1024 for all targets in the following commit:
https://git.musl-libc.org/cgit/musl/commit/?id=bc695a5ac1d7929e5c1ad5297eb47e146cccd157

Since loongarch64 requires musl 1.2.5, the expected value there is
different than the other targets, which still target an earlier musl.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

Author: Gelbpunkt

src/unix/linux_like/linux/musl/mod.rs

@@ -685,6 +685,7 @@ pub const __SIZEOF_PTHREAD_MUTEXATTR_T: usize = 4;
 pub const __SIZEOF_PTHREAD_RWLOCKATTR_T: usize = 8;
 pub const __SIZEOF_PTHREAD_BARRIERATTR_T: usize = 4;
 
+// FIXME(musl): Value is 1024 for all architectures since 1.2.4
 #[cfg(not(target_arch = "loongarch64"))]
 pub const CPU_SETSIZE: c_int = 128;
 #[cfg(target_arch = "loongarch64")]