seL4_libs: remove autoconf.h from seL4_libs

this commit removes autoconf.h generation from each lib and
append '#include <<lib_name>/gen_config.h>' after each
'#include <autoconf.h>' since autoconf.h is only the for the
kernel config now. This is a temporarily solution, since we
currently don't have a way to tell which header files each file
is trying to include when it includes 'autoconf.h'

sel4utils: use seL4_UserTop

This commit removes hard coded KERNEL_RESERVED_START values (which is
mean to represent the last page mappable by user-level) and calculates
the value from seL4_UserTop instead, such that if seL4_UserTop changes
at any point the vspace code will still work.

trivial: guard with pragma once

SELFOUR-1283: fix arch/sel4_arch sel4utils

- delete empty files and symlinks
- merge riscv sel4_arch and arch util.h