CREL dynamic relocations

[Fangrui Song <maskray@gcc.gnu.org>]

I have proposed a compact relocation format CREL at
https://groups.google.com/g/generic-abi/c/yb0rjw56ORw/m/eiBcYxSfAQAJ
(previously named RELLEB).

CREL primarily targets static relocations, achieving significant .o
file size reduction for lld builds: 18.0% for x86-64/aarch64 and 34.3%
for riscv64.
CREL holds promise for dynamic relocations as well, surpassing
Android's packed relocation format.

* R_*_GLOB_DAT and R_*_JUMP_SLOT relocations can typically be encoded
with repeated 0x09 0x01 (when shift==3, offset++, symidx++).
* Absolute relocation (__cxa_pure_virtual@CXXABI_1.3 + 0) relocations
typically require just one byte (offset+=n)

While CREL's benefit for non-relative relocations is smaller than
RELR's optimizing for relative relocations, it still shines for
programs where non-relative relocations make up more than 5% of the
file size
(quite a few GUI libraries, probably due to C++ virtual tables).
We could even consider phasing out REL/RELA for new architectures in
favor of CREL.

I'd love to hear your feedback on CREL and hope someone might consider
implementing rtld support (and binutils support if you like that as
well:) )
https://sourceware.org/bugzilla/show_bug.cgi?id=31541

I've notified binutils and GCC folks at
https://sourceware.org/pipermail/binutils/2024-March/133153.html
("CREL relocation format for ELF (was: RELLEB)").