scripts/glibcelf.py: Add *T_RISCV_* constants

SHT_RISCV_ATTRIBUTES, PT_RISCV_ATTRIBUTES, DT_RISCV_VARIANT_CC were
added in commit 0b6c6750
("Update RISC-V specific ELF definitions").  This caused the
elf/tst-glibcelf consistency check to fail.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>