[BOLT][RISCV] Recognize mapping symbols

The RISC-V psABI [1] defines them similarly to AArch64.

[1] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#mapping-symbol

Reviewed By: yota9, Amir

Differential Revision: https://reviews.llvm.org/D153277