Allow elf2flt to work with RISC-V 32-bit targets. With these changes, the
uclibc toolchain and busybox can work fine for rv32 no MMU systems with
no noticable problem.

Signed-off-by: Charles Lohr <lohr85@gmail.com>
[Rebased onto latest tree for upstreaming]
Signed-off-by: Jesse Taube <Mr.Bossman075@gmail.com>
[Add more ELF relco types and edit commit message]
Signed-off-by: Yimin Gu <ustcymgu@gmail.com>