loader: Ignore the .interp section on RISC-V

Without this we risk having the .interp section be placed earlier in the
file and mess with section offsets; in particular it has been seen to be
placed at the start of the file and cause the PE/COFF header to not be
at address 0. This is the same fix as was done for arm64 in r365578.

Reviewed by: mhorne, imp
Approved by: mhorne, imp
Differential Revision: https://reviews.freebsd.org/D27603