RISC-V: Error and warning messages tidy.

RISC-V: Error and warning messages tidy.

Error and warning messages usually starting with lower case letter,
and without the period at the end.  Besides, add the prefixed "internel:"
at the beginning of the messages when they are caused internally.
Also fix indents and typos.

bfd/
    * elfnn-riscv.c (riscv_merge_attributes): Fix typos of messages.
gas/
    * config/tc-riscv.c: Error and warning messages tidy.
    * testsuite/gas/riscv/priv-reg-fail-fext.l: Updated.
    * testsuite/gas/riscv/priv-reg-fail-read-only-01.l: Likewise.
    * testsuite/gas/riscv/priv-reg-fail-read-only-02.l: Likewise.
    * testsuite/gas/riscv/priv-reg-fail-rv32-only.l: Likewise.
    * testsuite/gas/riscv/priv-reg-fail-version-1p10.l: Likewise.
    * testsuite/gas/riscv/priv-reg-fail-version-1p11.l: Likewise.
    * testsuite/gas/riscv/priv-reg-fail-version-1p9p1.l: Likewise.
ld/
    * testsuite/ld-riscv-elf/attr-merge-priv-spec-failed-01.d: Updated.
    * testsuite/ld-riscv-elf/attr-merge-priv-spec-failed-02.d: Likewise.
    * testsuite/ld-riscv-elf/attr-merge-priv-spec-failed-03.d: Likewise.
    * testsuite/ld-riscv-elf/attr-merge-priv-spec-failed-04.d: Likewise.
    * testsuite/ld-riscv-elf/attr-merge-priv-spec-failed-05.d: Likewise.
    * testsuite/ld-riscv-elf/attr-merge-priv-spec-failed-06.d: Likewise.
opcodes/
    * riscv-dis.c (parse_riscv_dis_option): Fix typos of message.