feature_tests.h: always use largefile interfaces

feature_tests.h: always use largefile interfaces

this is transitional; we'll remove the non-64bit interfaces after
bootstrapping everything to use largefile.

remove the #error from libelf.h; the notes in libelf/README.LFS claim
that new interfaces using off64_t would have to be written to make
libelf large-file aware, but that's not true if off_t is always 64 bits
(which it is after the feature_tests.h change). do similarly for
old_procfs.h, swap.h, and ustat.h.

in 32-bit libc/libgen/madv, stop building the 32-bit interfaces. the
header pragmas should ensure everything ends up using the *64 symbols.
the resulting 32-bit libc will *only* have symbol names ending in "64",
and the 64-bit one has normal names. in rtld, apply a temporary
workaround that copes with the modified symbol names.

to bootstrap, first install the new headers onto your current system and
rebuild & install all of userland to make everything 32-bit there use
the *64 symbols. then build base.