Config LLVM with --enable-targets=arm , how to make Clang generate code for ARM Cortex-A8?

hi all
I use --enable-targets=arm configure LLVM, build it with Clang, but when I using Clang, it say:
#clang -triple /root/a.c

error: unable to create target: ‘No available targets are compatible with this
triple, see -version for the available targets.’

1 diagnostic generated.

How can I use Clang generate code for ARM Cortex-A8?
thank you