[RISCV] Set correct encodings for DWARF exception handling

This patch sets correct encodings for DWARF exception handling for RISC-V
(other than call site encoding, which must be udata4 rather than uleb128 and
is handled by D63415).

This has the same intend as D63409, except this version matches GCC/binutils
behaviour which uses the same encodings regardless of PIC/non-PIC and
medlow/medany code model.

llvm-svn: 366327