Add musl definitions for x86, arm and asmjs

I've tested x86 against C locally, but not arm or asmjs.
I added the arm definitions because asmjs's C is derived from arms.

Mysteriously, my locally-built musl does not contain a
definition for _SC_2_C_VERSION, so I just removed it.
diff --git a/src/unix/notbsd/mod.rs b/src/unix/notbsd/mod.rs
index 6c93a0d..ee123f0 100644
--- a/src/unix/notbsd/mod.rs
+++ b/src/unix/notbsd/mod.rs
@@ -1,6 +1,5 @@
 use dox::mem;
 
-pub type rlim_t = c_ulong;
 pub type sa_family_t = u16;
 pub type pthread_key_t = ::c_uint;
 pub type speed_t = ::c_uint;
@@ -108,7 +107,8 @@
         pub dli_saddr: *mut ::c_void,
     }
 
-    #[cfg_attr(any(target_arch = "x86", target_arch = "x86_64"),
+    #[cfg_attr(any(all(target_arch = "x86", not(target_env = "musl")),
+                   target_arch = "x86_64"),
                repr(packed))]
     pub struct epoll_event {
         pub events: ::uint32_t,
@@ -425,9 +425,6 @@
 
 pub const EFD_CLOEXEC: ::c_int = 0x80000;
 
-pub const F_SETLK: ::c_int = 6;
-pub const F_SETLKW: ::c_int = 7;
-
 pub const MNT_FORCE: ::c_int = 0x1;
 
 pub const Q_SYNC: ::c_int = 0x800001;