elftoolchain: fix an incorrect e_flags description

r349482 introduced the definitions and descriptions of the RISC-V
specific e_flags values to elftoolchain. However, the description for
the EF_RISCV_RVE flag was incorrectly duplicated from EF_RISCV_RVC. Fix
this by providing the proper description for this flag.

Reported by: jhb
Approved by: markj (mentor)