Fix for --gc-sections on NetBSD

Fix for --gc-sections on NetBSD

This patch fixes http://bugs.ntp.org/1844, works around
http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=40401, by
improving the test for support of --gc-sections to run a program which
reads a file using stdio built with --gc-sections, instead of simply
link the binary.  This catches the buggy linker as the garbage
collection removes a tag NetBSD uses to distinguish its own elf
binaries from Linux ones, causing it to treat conftest as a Linux
binary and run it with the wrong syscall table.