round_trip::elf::symtab_shndx fails on big endian

[plugwash]

round_trip::elf::symtab_shndx fails on big endian, I initially noticed this on a Debian CI test on s390x, but I also reproduced it manually with the latest version of the gimli-rs git repository on s390x, ppc64 and powerpc. The rustc used in all cases was the one from Debian sid.

---- round_trip::elf::symtab_shndx stdout ----
thread 'round_trip::elf::symtab_shndx' panicked at 'assertion failed: `(left == right)`
  left: `Section(SectionIndex(16711680))`,
 right: `Section(SectionIndex(65280))`', tests/round_trip/elf.rs:38:9

I bisected and it seems the symtab_shndx test started failing with commit 6b8c39cbdc73f24e76d080d1f41a6c5fb3f37624 though other round trip tests that do not fail with the latest version were failing before that.