RISC-V: Force variables to .data for code_elim.

RISC-V: Force variables to .data for code_elim.

RISC-V puts a global variable in .sdata by default, which causes the
add-symbol-file commands with -s .data to fail as there is no .data section.
This fixes 3 testsuite failures.

	gdb/testsuite/
	* gdb.base/code_elim.exp: For riscv, set additional_flags
	to include -msmall-data-limit=0.