icp: mark asm files with noexec stack

[perfinion]

Similar to commit a3600a1.
Asm files need an explicit note that they do not require an executable
stack.

This is similar to:
#4962

[mention-bot]

@perfinion, thanks for your PR! By analyzing the history of the files in this pull request, we identified @tonyhutter to be a potential reviewer.

[perfinion]

@behlendorf I haven't gotten around to getting a gentoo buildbot setup yet (its on my todo list). I just realized maybe we could just add a readelf thing to the tests. readelf is in binutils so should work on any distro. scanelf (from pax-utils) is even nicer but i dont think every distro has it packaged.

$ find . -name "*.o" -exec readelf -SW {} + | grep "^File\|GNU-stack"
File: ./asm-x86_64/sha2/sha512_impl.o
[ this has no output so it's wrong ]
File: ./icp.o
  [225] .note.GNU-stack   PROGBITS        0000000000000000 6da558 000000 00   X  0   0  0
[ the X here is the problem ]

[perfinion]

Looks like fedora has pax-utils too, in that case this is easier. if there are no problems the output is silent.

jason@meriadoc ~/code/kernel/zfs $ scanelf -qyRAF '%e %p' .
--X --- --- /module/icp/icp.ko
!WX --- --- /module/icp/asm-x86_64/sha2/sha512_impl.o
--X --- --- /module/icp/icp.o
!WX --- --- /lib/libicp/asm-x86_64/sha2/sha512_impl.o
!WX --- --- /lib/libicp/asm-x86_64/sha2/.libs/sha512_impl.o
!WX --- --- /lib/libicp/.libs/libicp.a:sha512_impl.o
RWX --- --- /lib/libzpool/.libs/libzpool.so.2.0.0
!WX --- --- /lib/libzpool/.libs/libzpool.a:sha512_impl.o
RWX --- --- /tests/zfs-tests/tests/functional/checksum/sha2_test

[behlendorf]

@perfinion it looks like pax-utils is in EPEL too which is nice because that means we can easily add it to the STYLE builder. How about opening a PR with adds a 'paxcheck' target to the Makefile similar to the cppcheck which is used by the lint target. This will make it very easy to add to the automated testing and make it easy to run locally.