discard .eh_frame sections

The .cfi_* assembler directives cause call frame information to be
emitted in a special .eh_frame section. But this is not needed in the
final binary, because we are not doing panic unwinding.

GNU ld already discards this section with its (default) --gc-sections
behaviour. Lld doesn't do that by default though, instead it complains:

    rust-lld: error: no memory region specified for section '.eh_frame'

So let's explicitly discard the .eh_frame section.