Makefile: Make sure ld is always aware of the target abi/emulation

On Makefile merge_objs calls ld without providing any hints
on the target so on a multilib toolchain for example it will
always pick the default one (elf64-littleriscv) which will
then result a failure when compiling on RV32 since the objects
will be 32bits and the target will be 64bits. The same happens
on compile_ld that calls gcc without CFLAGS so it doesn't
get mabi/march.

With this patch OpenSBI compiles on RV32 (PLATFORM_RISCV_XLEN=32),
I tested fw_jump.elf on qemu (but it doesn't boot the kernel yet
-bbl also doesn't boot the kernel so it may be something else).

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>