use an accessor function for __libc data pointer when compiled as PIC

prior to this change, a large portion of libc was unusable prior to
relocation by the dynamic linker, due to dependence on the global data
in the __libc structure and the need to obtain its address through the
GOT. with this patch, the accessor function __libc_loc is now able to
obtain the address of __libc via PC-relative addressing without using
the GOT. this means the majority of libc functionality is now
accessible right away.

naturally, the above statements all depend on having an architecture
where PC-relative addressing and jumps/calls are feasible, and a
compiler that generates the appropriate code.

src/internal/libc.c

 #include "libc.h"
 
-struct libc libc;
+#ifdef __PIC__
+struct __libc *__libc_loc()
+{
+	static struct __libc __libc;
+	return &__libc;
+}
+#else
+struct __libc __libc;
+#endif

src/internal/libc.h

@@ -4,8 +4,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 
-#define libc __libc
-extern struct libc {
+struct __libc {
 	void (*lock)(volatile int *);
 	void (*cancelpt)(int);
 	int (*atexit)(void (*)(void));
@@ -16,7 +15,15 @@ extern struct libc {
 	int (*rsyscall)(int, long, long, long, long, long, long);
 	void (**tsd_keys)(void *);
 	void (*fork_handler)(int);
-} libc;
+};
+
+#ifdef __PIC__
+extern struct __libc *__libc_loc(void) __attribute__((const));
+#define libc (*__libc_loc())
+#else
+extern struct __libc __libc;
+#define libc __libc
+#endif
 
 
 /* Designed to avoid any overhead in non-threaded processes */