RISC-V: Add path of library directories for the 32-bit

With RV32 support the list of possible RISC-V system directories
increases to:
     - /lib64/lp64d
     - /lib64/lp64
     - /lib32/ilp32d
     - /lib32/ilp32
     - /lib (only ld.so)

This patch changes the add_system_dir () macro to support the new ilp32d
and ilp32 directories for RV32.  While refactoring this code let's split
out the confusing if statements into a loop to make it easier to
understand and extend.

Reviewed-by: Maciej W. Rozycki <macro@wdc.com>