linux: fix cpu model parsing on newer arm kernels

The format of /proc/cpuinfo on ARM kernels >= 3.8 has changed. Scan for
the string "model name" (like x86) first, "Processor" second.

Fixes #812.
linux: fix memory leak in uv_cpu_info() error path

Any memory allocated to hold CPU model strings wasn't freed on error.
linux: don't ignore OOM errors in uv_cpu_info()