RISC-V: Handle multi-letter extension for multilib-generator

RISC-V: Handle multi-letter extension for multilib-generator

 - The order of multi-lib config could be wrong if multi-ltter are
   used, e.g. `./multilib-generator rv32izfh-ilp32--c`, would expect
   rv32ic_zfh/ilp32 reuse rv32i_zfh/ilp32, however the multi-ltter is not
   handled correctly, it will generate reuse rule for rv32izfhc/ilp32
   which is invalid arch configuration.

 - Remove re-use rule gen for g/imafd, because we canonicalize the -march at
   gcc driver too, so we don't need handle 'g' for multilib now.

gcc/ChangeLog:

	* config/riscv/multilib-generator (arch_canonicalize): Handle
	multi-letter extension.
	Using underline as separator between different extensions.