GCC 15 + LTO: cyclotomic exponentiation failures -> BLS signatures, KZG commitments, EIP-4844

[mratsim]

The following fails with GCC 15 + LTO, (tested with nim v2.0.8 and v2.2.8):

nim c -r -d:lto -d:release --outdir:build/test_suite  --nimcache:nimcache/tests/math_pairings/t_pairing_bn254_snarks_gt_exp.nim tests/math_pairings/t_pairing_cyclotomic_subgroup.nim

❯ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/15.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust,cobol --enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://gitlab.archlinux.org/archlinux/packaging/packages/gcc/-/issues --with-build-config=bootstrap-lto --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-libstdcxx-backtrace --enable-link-serialization=1 --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 15.2.1 20260209 (GCC)

Test passes without LTO or with Clang (LTO or not)

Cyclotomic exponentiation is a dependency for pairings, BLS signatures, KZG commitments, EIP-4844

[mratsim]

#596 works around by disabling LTO in CI.

However some downstream users might steal enable LTO so we need to fix this. Which might require an assembly code-generator to ensure independence from compiler (which is also good for constant-time and also ensuring no regression after audit)