RISC-V: Add assembly support for TLSDESC.

gas/
    * tc-riscv.c (percent_op_*): Add support for %tlsdesc_hi,
    %tlsdesc_load_lo, %tlsdesc_add_lo and %tlsdesc_call. percent_op_rtype
    renamed to percent_op_relax_only as this matcher is extended to handle
    jalr as well which is not R-type.
    (riscv_ip): Apply the percent_op_relax_only rename and update comment.
    (md_apply_fix): Add TLSDESC_* to relaxable list. Add TLSDESC_HI20 to
    TLS relocation check list.
    * testsuite/gas/riscv/tlsdesc.*: New test cases for TLSDESC relocation
    generation.
opcodes/
    * riscv-opc.c (riscv_opcodes): Add a new syntax for jalr with
    %tlsdesc_call annotations.