File::Spec::Unix->tmpdir: Always return an absolute path

This is generally a non-issue, however, if /tmp doesn't exist
and $ENV{TMPDIR} isn't set, ->tmpdir() used to return ".", which
broke the following pattern:

    use File::Temp qw(tempdir);
    use File::Spec;
    my $tmpdir = tempdir(CLEANUP => 1);
    chdir $tmpdir;
    my $file = File::Spec->catfile($tmpdir, "foo");
    open my $fh, ">", $file or die $!;

Because $tmpdir would be something like 'bfhskjf94589', and after
the chdir, the open() would've tried to open $tmpdir/$tmpdir/foo.
lib/ExtUtils/t/Embed.t: Skip tests is cross-compiling and $Config{cc} is not available
dist/ExtUtils-Install: Skip tests if make is not available
cpan/DB_File: Checks if tests should be skipped under PERL_CORE
cpan/Module-Build/t/lib/MBTest.pm, check_compiler: Return false if cross-compiling and !-e $Config{cc}
t/porting/: Skips for cross-compilation
cpan/ExtUtils-MakeMaker: Skips if make is not available
Makefile.SH, test_prep: Copy all the files needed by make test
cpan/ExtUtils-Constant/t/Constant.t: Skip tests if cross-compiling and make is not available
Win32: Skip tests if not on Windows
Skip the VMS tests if not on VMS