RISC-V: Fix riscv g++ testsuite EH failures.

RISC-V: Fix riscv g++ testsuite EH failures.

This fixes some EH failures for the medany code model in the g++ testsuite.
The problem is that the assembler is computing some values in the eh_frame
section as constants, that instead should have had relocs to be resolved by
the linker.  This happens in output_cfi_insn in the DW_CFA_advance_loc case
where it compares label frags and immediately simplifies if they are the
same.  We can fix that by forcing a new frag after every instruction
that the linker can reduce in size.  I've also added a testcase to verify
the fix.  This was tested with binutils make check, and gcc/g++ make checks on
qemu for medlow and medany code models.

	gas/
	* config/tc-riscv.c (append_insn): Call frag_wane and frag_new at
	end for linker optimizable relocs.
	* testsuite/gas/riscv/eh-relocs.d: New.
	* testsuite/gas/riscv/eh-relocs.s: New.
	* testsuite/gas/riscv/riscv.exp: Run eh-relocs test.