xtensa: Make register A0 allocable for the CALL0 ABI

xtensa: Make register A0 allocable for the CALL0 ABI

This patch offers an additional allocable register by RA for the CALL0
ABI.

> Register a0 holds the return address upon entry to a function, but
> unlike the windowed register ABI, it is not reserved for this purpose
> and may hold other values after the return address has been saved.
  - Xtensa ISA Reference Manual,
                   8.1.2 "CALL0 Register Usage and Stack Layout" [p.589]

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_conditional_register_usage):
	Remove register A0 from FIXED_REGS if the CALL0 ABI.
	(xtensa_expand_epilogue): Change to emit '(use (reg:SI A0_REG))'
	unconditionally after restoring callee-saved registers for
	sibling-call functions, in order to prevent misleading that
	register A0 is free to use.