[ELF] Add PT_RISCV_ATTRIBUTES program header

Close https://github.com/llvm/llvm-project/issues/63084

Unlike AArch32, RISC-V defines PT_RISCV_ATTRIBUTES to include the
SHT_RISCV_ATTRIBUTES section. There is no real-world use case yet.

We place PT_RISCV_ATTRIBUTES after PT_GNU_STACK, similar to PT_ARM_EXIDX. GNU ld
places PT_RISCV_ATTRIBUTES earlier, but the placement should not matter.

Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/71

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D152065