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

@@ -1,3 +1,11 @@
 #include "libc.h"
 
-struct libc libc;
+#ifdef __PIC__
+struct __libc *__libc_loc()
+{
+	static struct __libc __libc;
+	return &__libc;
+}
+#else
+struct __libc __libc;
+#endif