Make --export-dynamic work on non-PIC/PIE targets.

For CloudABI I'm only interested in generating non-PIC/PIE executables
on armv6 and i686, as PIE introduces larger overhead than on aarch64 and
x86_64. Still, I want to be able to instruct the linker to generate a
dynamic symbol table if requested. One example use for this is that
dynamic symbol tables can be used by programs to print nicely formatted
stacktraces, including symbol names.

Right now there seems to be some logic in LLD that it only wants to emit
dynamic symbol tables when either linking against libraries or when
building PIC. Let's extend this to also take --export-dynamic into
account.

Reviewed by:	ruiu
Differential Revision:	https://reviews.llvm.org/D29982

llvm-svn: 295240