Document .attribute reserved_registers

[kito-cheng]

This PR is based on #73, Tag_RISCV_reserved_registers is introduced by riscv-non-isa/riscv-elf-psabi-doc#195 .

• Adding new syntax for .attribute argument, accept register list style input:

RESERVED_REGS := '{' REG_LIST '}'

REG_LIST      := REG_LIST ',' REG_LIST
               | REG_RANGE
               | REG

REG_RANGE     := REG '-' REG

REG           := <register-name> | <abi-register-name> # e.g. x10, t3, f10 or fa2

For example: x6, x7, x8 and f10 are reserved, then the value of
Tag_RISCV_reserved_register is 0x400000001c1, and it also could be represent
as {x6, x7, x8, f10} or {x6-x8, f10}.