[RISCV] Make empty name symbols SF_FormatSpecific so that llvm-symbolizer...

[RISCV] Make empty name symbols SF_FormatSpecific so that llvm-symbolizer ignores them for symbolization

On RISC-V, clang emits empty name symbols used for label differences. (In GCC the symbols are typically `.L0`)
After D95916, the empty name symbols can show up in llvm-symbolizer's symbolization output.
They have no names and thus not useful. Set `SF_FormatSpecific` so that llvm-symbolizer will ignore them.

`SF_FormatSpecific` is also used in LTO but that case should not matter.

Corresponding addr2line problem: https://sourceware.org/bugzilla/show_bug.cgi?id=27585

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D98669