.sched.text: port this sub-section to section ranges

This ports the the sub-section .sched.text to use
the more standard section range interface, doing so
avoids mucking with the linker script and pushes us
to explicitly annotate the standard section where this
belongs, all in C code and header files.

The section range used, in this case "sched_text", is
arbitrary but we needs to be unique within the defined
section used, in this case SECTION_TEXT cannot have any
oclaredther "sched_text" section ranges.

We now have:

$ readelf -S kernel/locking/mutex.o | grep sched
  [ 6] .text.tbl.sched_t PROGBITS         0000000000000000  000000b0
  [ 9] .text.tbl.sched_t PROGBITS         0000000000000000  00000138
  [10] .text.tbl.sched_t PROGBITS         0000000000000000  00000138

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>