Update generate_target_board

RISCV: Add target_board generation for multiple arch_abi parameters
Currently this script just can handle one parameter for arch_abi,but there is more than one arch_abi in "newlib_multilib_names" which comes from the configure.

Execute the following command:
generate_target_board --sim-name riscv-sim --cmodel medany --build-arch-abi "rv32imafc-ilp32f rv64imafdc-lp64d"

Before this patch the result is:
 riscv-sim/-march=rv32imafc/-mabi=ilp32f rv64imafdc/-mcmodel=medany    
After this patch,the result is:
riscv-sim/-march=rv32imafc/-mabi=ilp32f/-mcmodel=medany
riscv-sim/-march=rv64imafdc/-mabi=lp64d/-mcmodel=medany
    
At the same time,there is small modification for Makefile.in,the
parameters of build-arch-abi should be enclosed in double quotation marks.
--build-arch-abi "$(NEWLIB_MULTILIB_NAMES)"

Signed-off-by: ff520git <wangfeng@eswincomputing.com>