RISC-V: Fix loss of function to script 'multilib-generator'

The arch 'rv32imac' will not be created when excuting
'./multilib-generator rv32imc-ilp32--a'

The output is:
MULTILIB_OPTIONS = march=rv32imc mabi=ilp32
MULTILIB_DIRNAMES = rv32imc ilp32
MULTILIB_REQUIRED = march=rv32imc/mabi=ilp32
MULTILIB_REUSE =

Analysis : The alts:['rv32imc', 'rv32imac'] will change
to ['rv32imac', 'rv32imc'] through function:unique(alts) processing,
This is the wrong alts should not be changed.
This patch fix it.

gcc/ChangLog:
	* config/riscv/multilib-generator: Adjusting the loop of 'alt' in 'alts'.

Signed-off-by: Songhe Zhu <zhusonghe@eswincomputing.com>