support configurable page size on mips, powerpc and microblaze

PAGE_SIZE was hardcoded to 4096, which is historically what most
systems use, but on several archs it is a kernel config parameter,
user space can only know it at execution time from the aux vector.

PAGE_SIZE and PAGESIZE are not defined on archs where page size is
a runtime parameter, applications should use sysconf(_SC_PAGE_SIZE)
to query it. Internally libc code defines PAGE_SIZE to libc.page_size,
which is set to aux[AT_PAGESZ] in __init_libc and early in __dynlink
as well. (Note that libc.page_size can be accessed without GOT, ie.
before relocations are done)

Some fpathconf settings are hardcoded to 4096, these should be actually
queried from the filesystem using statfs.
diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
index 6c841be..d26f252 100644
--- a/src/thread/pthread_create.c
+++ b/src/thread/pthread_create.c
@@ -1,5 +1,6 @@
 #include "pthread_impl.h"
 #include "stdio_impl.h"
+#include "libc.h"
 #include <sys/mman.h>
 
 static void dummy_0()