Enable -shared support for embedded RISC-V targets

Shared library support is incomplete for the RISC-V embedded elf
targets, 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

Reviewed by: philip
Approved by: philip
Differential Revision: https://reviews.freebsd.org/D25735