Remove sysdeps/x86_64/x32/dl-machine.h by folding x32 ARCH_LA_PLTENTER,
ARCH_LA_PLTEXIT and RTLD_START into sysdeps/x86_64/dl-machine.h. There
are no regressions on x86-64 nor x32. There are no changes in x86-64
_dl_start_user. On x32, _dl_start_user changes are