Support merging of .riscv.attributes sections

Documentation of the section format:
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#layout-of-riscvattributes-section

Example of the section content:
```
Attribute Section: riscv
File Attributes
  Tag_RISCV_stack_align: 16-bytes
  Tag_RISCV_arch: "rv64i2p1_m2p0_a2p1_f2p2_d2p2_c2p0_zicsr2p0_zifencei2p0_zmmul1p0_zaamo1p0_zalrsc1p0"
```