vfs: use 64bit size_t

Issue #1246
vfs: use 64bit off_t

Issue #1246
vfs: rename Vfs::off_t to Vfs::file_offset

Issue #1246
vfs: rename Vfs::size_t to Vfs::file_size

Issue #1246
vbox: enable auto test for 32bit Genode host

Issue #1246
libc: enable and fix warnings in backends

Issue #1246
fixup "libc: enable and fix warnings in backends"

Issue #1246