RISC-V: The 'multilib-generator' enhancement.

RISC-V: The 'multilib-generator' enhancement.

Think about this case:
  ./multilib-generator rv32imc-ilp32-rv32imac,rv32imacxthead-f
Here are 2 problems:
  1. A unexpected 'xtheadf' extension was made.
  2. The arch 'rv32imac' was not be created.
This modification fix these two, and also sorts 'multi-letter'.

gcc/ChangeLog:
	* config/riscv/arch-canonicalize (longext_sort): New function for
	 sorting 'multi-letter'.
	* config/riscv/multilib-generator: Adjusting the loop of 'alt' in
	'alts'.	The 'arch' may not be the first of 'alts'.
	(_expand_combination): Add underline for the 'ext' without '*'.
	This is because, a single-letter extension can always be treated well
	with a '_' prefix, but it cannot be separated out if it is appended
	to a multi-letter.