perf symbol: Add RISCV case in get_plt_sizes

perf symbol: Add RISCV case in get_plt_sizes

According to RISC-V psABI specification, the PLT (Program Linkage Table)
has the following layout:
- The first PLT entry occupies two 16-byte entries (32 bytes total)
- Subsequent PLT entries take up 16 bytes each

This aligns with the binutils-gdb implementation which defines the same
PLT sizes for RISC-V architecture.

Update get_plt_sizes() to set plt_header_size=32 and plt_entry_size=16
for EM_RISCV, matching the architecture's standard ABI.

Since AARCH64, LOONGARCH, and RISCV have the same PLT size definition,
they are merged together.

Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elfnn-riscv.c
Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>