provide generic way for sanity_check to look in both lib and lib64 for library files

[torbjoernk]

AFAIK the only way of checking whether library files ended up in lib/ or lib64/ is via

sanity_check_paths = {
  'files': [
    ('lib/myLib.so', 'lib64/myLib.so'),
    ('lib/myLib.a', 'lib64/myLib.a')
  ]
}

Wouldn't it be nice having this collapsing into something like

sanity_check_paths = {
  'files': [
    LibraryPathBuilder('myLib', shared=True, static=True)
  ]
}

It's less error prone to typos and way less tedious.

[boegel]

EasyBuild will now (since v3.6.1) automatically consider lib64/... paths whenever a lib/... is specified (and vice versa), see #2477, thanks again for the idea @torbjoernk!