Avoid forking for processor_count if possible

This re-orders the checks for the processor count so that forking is
only done when needed. alpha-linux does not support the processor entry
in /proc/cpuinfo so we must catch that case and fall back to using nproc
which does work on alpha.

This is a rework of ruby-concurrency#549 based on suggestions made in ruby-concurrency#576

lib/concurrent/utility/processor_counter.rb

@@ -85,10 +85,10 @@ def compute_processor_count
             result = WIN32OLE.connect("winmgmts://").ExecQuery(
               "select NumberOfLogicalProcessors from Win32_Processor")
             result.to_enum.collect(&:NumberOfLogicalProcessors).reduce(:+)
+          elsif File.readable?("/proc/cpuinfo") && (cpuinfo_count = IO.read("/proc/cpuinfo").scan(/^processor/).size) > 0
+            cpuinfo_count
           elsif File.executable?("/usr/bin/nproc")
             IO.popen("/usr/bin/nproc --all", &:read).to_i
-          elsif File.readable?("/proc/cpuinfo")
-            IO.read("/proc/cpuinfo").scan(/^processor/).size
           elsif File.executable?("/usr/bin/hwprefs")
             IO.popen("/usr/bin/hwprefs thread_count", &:read).to_i
           elsif File.executable?("/usr/sbin/psrinfo")