linux: use /proc/cpuinfo for CPU frequency

Obtain the CPU frequency from /proc/cpuinfo because there may not be any
cpufreq info available in /sys. This also means that the reported CPU speed
from now on is the *maximum* speed, not the *actual* speed the CPU runs at.

This change only applies to x86 because ARM and MIPS don't report that
information in /proc/cpuinfo.

Fixes joyent#588.

This is a back-port of commit 775064a from the master branch.

linux: use /proc/cpuinfo for CPU frequency

Obtain the CPU frequency from /proc/cpuinfo because there may not be any
cpufreq info available in /sys. This also means that the reported CPU speed
from now on is the *maximum* speed, not the *actual* speed the CPU runs at.

This change only applies to x86 because ARM and MIPS don't report that
information in /proc/cpuinfo.

Fixes joyent#588.

This is a back-port of commit 775064a from the master branch.

unix: support signal handlers outside the main loop

windows: fix handle leak in uv_fs_utime

windows: don't blow up when an invalid FD is used

windows: don't blow up when an invalid FD is used

darwin: emit relative path in fsevents

windows: fix memory corruption when closing shared server sockets

test: add test for uv_is_active and uv_is_closing

build: rework -fvisibility=hidden detection

Make the gcc_version macro conform with what node.js and v8 use. Important
because node.js's common.gypi is going to export it soon.