This commit backports the LLVM commit that adds support for the
`--update-section` flag to llvm-objcopy. We use this feature of GNU
objcopy to embed the symbol map in the kernel.
The corresponding LLVM Phabricator Differential Revision can be found
here: https://reviews.llvm.org/D112116
This patch is identical to the upstream commit, except for two hunks
that had to be changed as they didn't apply cleanly.
