Preprocessing not compatible with Linux arm64

[sim642]

In opam-repository CI with Linux arm64, there were failures for the 2.4.0 release like:

cpp: error: unrecognized command-line option '-m64'

Apparently the -m64 and -m32 options are specific to x86 GCC: https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html#index-m32-2.

On arm64 they might be -mabi=ilp32 and -mabi=lp64: https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html#index-mabi.
Somehow on M1 they aren't?!

The difficult thing is probably detecting the architecture reliably. Or maybe we should just try calling cpp to see if it errors or not.