RISC-V: Promote type correctly for libcalls

 - argument and return value for libcall won't promote at
   default_promote_function_mode_always_promote, however we expect it
   should sign-extend as normal function.

 - Witout this patch, this test case will fail at -march=rv64i -mabi=lp64.

 - The implementation of riscv_promote_function_mode is borrowed from MIPS.

gcc/ChangeLog

	* config/riscv/riscv.c (riscv_promote_function_mode): New.
	(TARGET_PROMOTE_FUNCTION_MODE): Use riscv_promote_function_mode.

gcc/testsuite/ChangeLog

	* gcc.target/riscv/promote-type-for-libcall.c: New.

From-SVN: r274107