readelf: Add RISC-V DWARF register aliases

This allows DWARF debugging output to use the common register
mneumonics, such as ra, sp, or t0.

DWARF registers 0-31 are mapped to the 32 general purpose registers,
which are then followed by the 32 floating point registers.

Reviewed by: markj
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D20779