UI test `multiple-reprs` fails on targets that set `c-enum-min-bits`

[japaric]

This part of the test assumes that the enum's discriminant has a size of c_int which is true for most targets but not for targets that set TargetOptions.c_enum_min_bits like thumbv7em-none-eabihf. The multiple-reprs test fails when run on those target.

affected revision: b5741a3

I'll submit a PR fixing the test on those targets shortly