kexec-tools-2.0.16 fails to compile here.
There was a similar fix for -mindirect-branch but it seems -mfunction-return was forgotten.
The thunk-inline version was made to work with '-mcmodel=large', so instead of filter-flags it could replace it with thunk-inline like this:
replace-flags -mfunction-return=thunk -mfunction-return=thunk-inline
replace-flags -mindirect-branch=thunk -mindirect-branch=thunk-inline
The only issue I see is that thunk-inline is slower than thunk.
At least it would be nice if the ebuilds did not filter thunk-inline (only thunk instead of thunk*) since that compiles.
Problem is hidden in >=sys-apps/kexec-tools-2.0.19 which just ignores user-specified CFLAGS during relevant part of building: bug #692930.