The stage before `xgcc` creates the first compiled patchelf
(i.e. not from bootstrapFiles).
The `xgcc` stage was inadvertently switching *back* to using the
patchelf *from* the bootstrapFiles.
The first commit in this PR adds self-checking comments (assertions)
to make it clear where each stage's patchelf comes from.
The second commit fixes the bug, and updates the self-checking
comments.
1847b1bc15