ELF: add version script void function test case

After r280733 we use LLVM's demangler in lld. As a result we no longer
have a discrepancy between f() and f(void) on FreeBSD (due to an issue
with FreeBSD's system demangler).

Restore a test case for a void arg function.

Differential Revision:	https://reviews.llvm.org/D24305

llvm-svn: 280831