Fixup versioning for simics APIs and add versioned tests

[novafacing]

Fixes up versioning of API functions to use a standard version-oriented cfg() system instead of the semi-bespoke approach used right now.

[novafacing]

This PR now also replaces the use of patchelf in the Fedora 20 builder image with an updated linker (builds ldd 5.0.2 from source).

Some motivation for posterity:

The original motivation for using patchelf is that ld.bfd on Fedora 20 interpreted cargo:rustc-link-lib and cargo:rustc-link-search directives in an unexpected way, resulting in a DT_NEEDED entry /absolute/path/to/libsimics-common.so. We used patchelf to replace this entry with libsimics-common.so, but this operation corrupted the ELF file in such a way that newer (verified on 2.38) versions of ld.so would load it correctly, but older (e.g. glibc 2.17) versions would not, complaining about misaligned ELF headers/instructions. This works fine for any modern system, but some users are on legacy systems. Removing the use of patchelf and using ld.ldd v5.0.2 fixes the issue, because ld.ldd 5.0.2 correctly emits those DT_NEEDED sections in the first place.

[tklengyel]

Couple minor things, otherwise LGTM