Add human-readable abi_imports and abi_exports build artefacts

[ermo]

The format below has the nice property that git will use the RFC822 header as its "anchor" for changes under it, thus making it easier to check at a glance what has changed.

libFlac.so.8:
<\t>FLAC_API_SUPPORTS_OGG_FLAC
<\t>FLAC__ChannelAssignmentString
<\t>FLAC__ENTROPY_CODING_METHOD_PARTITIONED_RICE2_ESCAPE_PARAMETER
<\t>FLAC__ENTROPY_CODING_METHOD_PARTITIONED_RICE2_PARAMETER_LEN
<\t>FLAC__ENTROPY_CODING_METHOD_PARTITIONED_RICE_ESCAPE_PARAMETER
libFOO.so.7:
<\t>FOO_BAR
<\t>FOO_BAZ

[ermo]

I've made some progress here in that I've re-enabled the elf-d/scanelf build target, such that we can use that as a proving ground for finalising the properties we want.