[PATCH v3] RISC-V: Enable dead code elimination

[Bin Meng]

> Select CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION for RISC-V, allowing
> the user to enable dead code elimination. In order for this to work,
> ensure that we keep the alternative table by annotating them with KEEP.
> 
> This boots well on qemu with both rv32_defconfig & rv64 defconfig, but
> it only shrinks their builds by ~1%, a smaller config is thereforce
> customized to test this feature:
> 
>           | rv32                   | rv64
>   --------|------------------------|---------------------
>    No DCE | 4460684                | 4893488
>       DCE | 3986716                | 4376400
>    Shrink |  473968 (~10.6%)       |  517088 (~10.5%)
> 
> The config used above only reserves necessary options to boot on qemu
> with serial console, more like the size-critical embedded scenes:
> 
>   - rv64 config: https://pastebin.com/crz82T0s
>   - rv32 config: rv64 config + 32-bit.config
> 
> Signed-off-by: Zhangjin Wu <falcon at tinylab.org>
> ---
>  arch/riscv/Kconfig              | 1 +
>  arch/riscv/kernel/vmlinux.lds.S | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)

Tested-by: Bin Meng <bmeng at tinylab.org>