kernel: Include objtool in kernel-devel

[sam-aws]

Issue number:
#819

Description of changes:
objtool is a requirement for compiling external modules, particularly if
stack validation is enabled at build time.

Testing done:
Built an image and compiled a module against the installed kernel headers from within the SDK container:

# docker run -it --net host --mount type=bind,source=/home/ANT.AMAZON.COM/samjonas/workbench/thar/module/mod,target=/tmp/mod --mount type=bind,source=/home/ANT.AMAZON.COM/samjonas/workbench/thar/module/kernels,target=/usr/src/kernels --mount type=bind,source=/home/ANT.AMAZON.COM/samjonas/workbench/thar/module/modules,target=/lib/modules bottlerocket/sdk-x86_64:v0.10.0
[builder@u87e72aa3c6c25c /]$ cd /tmp/mod
[builder@u87e72aa3c6c25c mod]$ make -C /lib/modules/5.4.16/build/ M=`pwd` modules
make: Entering directory '/usr/src/kernels/5.4.16'
  CC [M]  /tmp/mod/KMOD_XXX.o
  AS [M]  /tmp/mod/KMOD_YYY.o
  LD [M]  /tmp/mod/KMOD_ZZZ.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC [M]  /tmp/mod/KMOD_ZZZ.mod.o
  LD [M]  /tmp/mod/KMOD_ZZZ.ko
make: Leaving directory '/usr/src/kernels/5.4.16'
[builder@u87e72aa3c6c25c mod]$ echo $?
0

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

Signed-off-by: Samuel Mendoza-Jonas samjonas@amazon.com

[sam-aws]

@bcressey reminded me that objtool is probably compiled in the host arch, not the target arch - and indeed this breaks the build when cross compiling. Fix to come!

[sam-aws]

Updated to only include objtool for x86. After some poking around it seems objtool support is almost but not quite supported on arm64 (e.g. https://lore.kernel.org/linux-arm-kernel/20200109160300.26150-1-jthierry@redhat.com/)