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

stand/efi/loader/arch/riscv/ldscript.riscv

@@ -82,6 +82,7 @@ SECTIONS
   _edata = .;
 
   /* Unused sections */
+  .interp	: { *(.interp) }
   .dynstr	: { *(.dynstr) }
   .hash		: { *(.hash) }
 }