Fix instances of '#if __i386__'.

Triggers -Wundef, which is on in -Weverything.

Bug: http://b/31496165
Change-Id: Ie2241b19abd6257bebf68baa3ecc4de8066c722e

libc/bionic/pthread_create.cpp

@@ -44,7 +44,7 @@
 #include "private/ScopedPthreadMutexLocker.h"
 
 // x86 uses segment descriptors rather than a direct pointer to TLS.
-#if __i386__
+#if defined(__i386__)
 #include <asm/ldt.h>
 void __init_user_desc(struct user_desc*, bool, void*);
 #endif

libc/include/sys/user.h

@@ -38,7 +38,7 @@ __BEGIN_DECLS
 #define PAGE_SIZE 4096
 #define PAGE_MASK (~(PAGE_SIZE - 1))
 
-#if __i386__
+#if defined(__i386__)
 
 struct user_fpregs_struct {
   long cwd;