ACPI / PM: Merge do_suspend_lowlevel() into acpi_save_state_mem()

The function do_suspend_lowlevel() is specific to x86 and defined in
assembly code, so it should be called from the x86 low-level suspend
code rather than from acpi_suspend_enter().

Merge do_suspend_lowlevel() into the x86's acpi_save_state_mem() and
change the name of the latter to acpi_suspend_lowlevel(), so that the
function's purpose is better reflected by its name.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>