devel/binutils: enable shared for riscv64_none_elf

[mhorne]

Summary

Shared library support is incomplete for the RISC-V embedded elf target,
and was disabled some time ago [1]. However, it appears that it is
functional enough to support basic usage, and is required to
successfully build u-boot for RISC-V. This toolchain doesn't have
widespread usage, so any risk of re-enabling this support is outweighed
by the benefit of being able to build important ports like u-boot.
Override this default.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=be1dce26256aa9f6c7742e81c8c3db9eacff079d

Test Plan

The linked commit doesn't offer much of a description, but I spoke with
the author who told me a subset of unit tests fail for this target, so
he simply disabled the shared library support until someone motivated
comes along to address them.

I've been running this patch locally for several months, and u-boot builds and runs fine.

[mhorne]

Hi, any opposition to this? I will need at least one ports/maintainer approval in order to commit.

[philip]

I still have a lukewarm ports commit bit. :)

This looks good. Approved.