t/lib/MBTest.pm: When cross-compiling, check the compiler even if PERL_CORE=1

This is because the target system might not have a compiler at all,
or might have a compiler/ccflags combination that doesn't mesh well,
so we can't assume anything.
M::B::Unix::_detildefy: Protect against platforms without getpw{nam,uid}

Unfortunately there is no %Config key to check, so we have to
eval {} the calls; We also make the getpwuid case default to glob("~").