[PATCH RFC v2] Add support for non-contiguous memory regions

[Christophe Lyon]

Hi,

This is a follow-up to
https://sourceware.org/ml/binutils/2019-11/msg00402.html

This version detects cases it isn't (yet?) able to handle and emits an
error message (and uses abort() which can probably be made more
user-friendly).

Specifically, I have:
* improved the testcase in ld-elf, hoping that it's now generic enough
(at least it passes on arm and powerpc).

* added a powerpc testcase, based on the sample provided by Simon. It
now fails with an error message when relaxation was applied.

* added several arm testcases, some of which involve farcall stubs
insertion. Something annoying with stubs handling is that I had to
modify arm-specific code to generate an error in a case where the stub
destination could not be allocated. We'd have to do this in every
target that has XXX_build_one_stub().
I now add the SEC_LINKER_CREATED flag on the stubs sections so that we
can emit a helpful error message in case the stubs do not fit in
memory.

There's a new test to check that section order does not change.

Finally, I had to change the way I remove input_section statements in
lang_size_sections_1() to correctly handle the case where we have to
remove the head of the list.

There are now 4 patches, to hopefully make review/comments easier:
* patch1: is the main (code) patch
* patch2: generic test
* patch3: arm tests
* patch4: powerpc test

Thoughts?

Thanks,

Christophe
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