[RISCV] Make mapping symbols SF_FormatSpecific

This ensures that llvm-symbolizer ignores them for symbolization.

Note: unlike aarch64-mapping-symbol.s, the test included here does not
test if the mapping symbols are actually in the symbol table. The reason
is that llvm-mc support for RISC-V mapping symbols (D153260) has not
landed yet, so the mapping symbols simply aren't there. However, D153260
would like to depend on this patch together with D156190 to avoid having
to update a large amount of tests.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D156236