Enable a few more pthread* files (pthread_key*, pthread_once) (WebAssembly#348)

* threads: enable TSD functions

* threads: Enable pthread_once

Author: loganek

Makefile

@@ -242,7 +242,9 @@ LIBC_TOP_HALF_MUSL_SOURCES += \
         thread/pthread_condattr_setclock.c \
         thread/pthread_condattr_setpshared.c \
         thread/pthread_create.c \
+        thread/pthread_getspecific.c \
         thread/pthread_join.c \
+        thread/pthread_key_create.c \
         thread/pthread_mutex_consistent.c \
         thread/pthread_mutex_destroy.c \
         thread/pthread_mutex_init.c \
@@ -257,6 +259,7 @@ LIBC_TOP_HALF_MUSL_SOURCES += \
         thread/pthread_mutexattr_setpshared.c \
         thread/pthread_mutexattr_setrobust.c \
         thread/pthread_mutexattr_settype.c \
+        thread/pthread_once.c \
         thread/pthread_rwlock_destroy.c \
         thread/pthread_rwlock_init.c \
         thread/pthread_rwlock_rdlock.c \
@@ -270,6 +273,7 @@ LIBC_TOP_HALF_MUSL_SOURCES += \
         thread/pthread_rwlockattr_init.c \
         thread/pthread_rwlockattr_setpshared.c \
         thread/pthread_setcancelstate.c \
+        thread/pthread_setspecific.c \
         thread/pthread_self.c \
         thread/pthread_testcancel.c \
         thread/sem_destroy.c \