module: break nested ARCH_HAS_STRICT_MODULE_RWX and STRICT_MODULE_RWX #ifdefs

Various frob_* and module_{enable,disable}_* functions are defined in a
CONFIG_ARCH_HAS_STRICT_MODULE_RWX ifdef block which also has a nested
CONFIG_STRICT_MODULE_RWX ifdef block within it. This is unecessary and
makes things hard to read. Not only that, this construction requires
redundant empty stubs for module_enable_nx(). I suspect this was
originally done for cosmetic reasons - to keep all the frob_* functions
in the same place, and all the module_{enable,disable}_* functions right
after, but as a result it made the code harder to read.

Make this more readable by unnesting the ifdef blocks and getting rid of
the redundant empty stubs.

Signed-off-by: Jessica Yu <jeyu@kernel.org>