Enable big-endian suffixed dynamic linker per glibc multi-abi support.

And to avoid a future churn and version pairingi hassles, also allow
arc700 although glibc for ARC currently doesn't support it.

gcc/
xxxx-xx-xx  Vineet Gupta <vgupta@synopsys.com>

       * config/arc/linux.h: GLIBC_DYNAMIC_LINKER support BE/arc700