Don't fail to disassemble illegal instructions
Sync riscv-opc.h with riscv-opcodes
Make mul/div cost model more realistic
Only emit fdiv/fsqrt if -mfdiv is specified

No implementations we're aware of bake these into HW, so this is faster.
Improve cost model for generating integer constants
Fix linker relaxation against local symbols
Fix sigaction
Add several syscalls
syscalls added (openat, access, accessat, faccessat, etc.)