[stdlib stubs] Fix up Linux build to know about pthread types.

Adds in Linux platform support for our pthread TLS. Replace usage of
PTHREAD_KEYS_MAX with a sentinel value, as it's tricky to define
cross-platform and was only lightly used inside sanity checks.

Author: milseman

Diff for: stdlib/public/SwiftShims/LibcShims.h

@@ -133,10 +133,6 @@ int _swift_stdlib_pthread_setspecific(
   __swift_pthread_key_t key, const void * _Nullable value
 );
 
-SWIFT_RUNTIME_STDLIB_INTERFACE
-__swift_pthread_key_t _swift_stdlib_PTHREAD_KEYS_MAX(void);
-
-
 // TODO: Remove horrible workaround when importer does Float80 <-> long double.
 #if (defined __i386__ || defined __x86_64__) && !defined _MSC_VER
 static inline SWIFT_ALWAYS_INLINE

Diff for: stdlib/public/core/ThreadLocalStorage.swift

@@ -117,10 +117,11 @@ internal func _destroyTLS(_ ptr: UnsafeMutableRawPointer?) {
 
 // Lazily created global key for use with pthread TLS
 internal let _tlsKey: __swift_pthread_key_t = {
-  var key: __swift_pthread_key_t = _swift_stdlib_PTHREAD_KEYS_MAX()
+  let sentinelValue = __swift_pthread_key_t.max
+  var key: __swift_pthread_key_t = sentinelValue
   let success = _swift_stdlib_pthread_key_create(&key, _destroyTLS)
   _sanityCheck(success == 0, "somehow failed to create TLS key")
-  _sanityCheck(key != _swift_stdlib_PTHREAD_KEYS_MAX(), "Didn't make a new key")
+  _sanityCheck(key != sentinelValue, "Didn't make a new key")
   return key
 }()
 

Diff for: stdlib/public/stubs/LibcShims.cpp

@@ -18,6 +18,8 @@
 #else
 #include <unistd.h>
 #endif
+#include <pthread.h>
+
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -125,11 +127,6 @@ int swift::_swift_stdlib_pthread_setspecific(
   return pthread_setspecific(key, value);
 }
 
-SWIFT_RUNTIME_STDLIB_INTERFACE
-__swift_pthread_key_t _swift_stdlib_PTHREAD_KEYS_MAX(void) {
-  return PTHREAD_KEYS_MAX;
-}
-
 #if defined(__APPLE__)
 #include <malloc/malloc.h>
 SWIFT_RUNTIME_STDLIB_INTERFACE