Be more selective about skipped pthread on musl tests

Fixup the value of PTHREAD_STACK_MIN as well

Author: alexcrichton

libc-test/build.rs

@@ -200,7 +200,7 @@ fn main() {
             "SIG_IGN" => true, // sighandler_t weirdness
 
             // types on musl are defined a little differently
-            n if musl && n.contains("PTHREAD") => true,
+            n if musl && n.contains("__SIZEOF_PTHREAD") => true,
 
             _ => false,
         }

src/unix/notbsd/linux/mod.rs

@@ -221,7 +221,9 @@ extern {
 }
 
 cfg_if! {
-    if #[cfg(any(target_arch = "arm", target_arch = "x86",
+    if #[cfg(any(target_env = "musl"))] {
+        pub const PTHREAD_STACK_MIN: ::size_t = 2048;
+    } else if #[cfg(any(target_arch = "arm", target_arch = "x86",
                  target_arch = "x86_64"))] {
         pub const PTHREAD_STACK_MIN: ::size_t = 16384;
     } else {