Renaming of cv.slet to cv.sle

For issue [#100](https://github.com/openhwgroup/corev-binutils-gdb/issues/100)

Files Changes:

  * gas/testsuite/gas/riscv/cv-alu-slet.d: Changed the name of cv.slet.
  * gas/testsuite/gas/riscv/cv-alu-slet.s: Likewise.
  * gas/testsuite/gas/riscv/cv-alu-sletu.d: Likewise.
  * gas/testsuite/gas/riscv/cv-alu-sletu.s: Likewise.
  * opcodes/riscv-opc.c: Likewise.

Signed-off-by: Mary Bennett <mary.bennett@embecosm.com>