perf symbols: Ignore mapping symbols on riscv

perf symbols: Ignore mapping symbols on riscv

RISCV ELF use mapping symbols with special names $x, $d to
identify regions of RISCV code or code with different ISAs[1].
These symbols don't identify functions, so will confuse the
perf output.

The patch filters out these symbols at load time, similar to
"4886f2ca perf symbols: Ignore mapping symbols on aarch64".

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

Signed-off-by: Haibo Xu <haibo1.xu@intel.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20250409025202.201046-1-haibo1.xu@intel.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>