module: treat exit sections the same as init sections when !CONFIG_MODULE_UNLOAD

Dynamic code patching (alternatives, jump_label and static_call) can
have sites in __exit code, even it __exit is never executed. Therefore
__exit must be present at runtime, at least for as long as __init code
is.

Additionally, for jump_label and static_call, the __exit sites must also
identify as within_module_init(), such that the infrastructure is aware
to never touch them after module init -- alternatives are only ran once
at init and hence don't have this particular constraint.

By making __exit identify as __init for MODULE_UNLOAD, the above is
satisfied.

So, when !CONFIG_MODULE_UNLOAD, the section ordering should look like the
following, with the .exit sections moved to the init region of the module.

Core section allocation order:
 	.text
 	.rodata
 	__ksymtab_gpl
 	__ksymtab_strings
 	.note.* sections
 	.bss
 	.data
 	.gnu.linkonce.this_module
 Init section allocation order:
 	.init.text
 	.exit.text
 	.symtab
 	.strtab

[jeyu: thanks to Peter Zijlstra for most of changelog]

Link: https://lore.kernel.org/lkml/YFiuphGw0RKehWsQ@gunter/
Link: https://lore.kernel.org/r/20210323142756.11443-1-jeyu@kernel.org


Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jessica Yu <jeyu@kernel.org>