/proc/cpuinfo parsing crashes on s390x

[pgier]

The current logic for parsing /proc/cpuinfo is written for x86 platforms, but doesn't really handle other formats currently. The worst case is probably s390x which causes an array index out of bounds panic.

goroutine 39 [running]:
github.com/prometheus/procfs.parseCPUInfo(0xc000203000, 0x4c3, 0xe00, 0x4c3, 0xe00, 0x0, 0x0, 0x1)
	/go/src/github.com/prometheus/node_exporter/vendor/github.com/prometheus/procfs/cpuinfo.go:85 +0x1d32
github.com/prometheus/procfs.FS.CPUInfo(0x3ffcbd7f550, 0xa, 0x24000000000f4684, 0x1d394, 0xc0001eb6c0, 0x70, 0x70)
	/go/src/github.com/prometheus/node_exporter/vendor/github.com/prometheus/procfs/cpuinfo.go:61 +0x180
github.com/prometheus/node_exporter/collector.(*cpuCollector).updateInfo(0xc00005d180, 0xc0000de180, 0x32ea661f00000000, 0x325e45f98388)
	/go/src/github.com/prometheus/node_exporter/collector/cpu_linux.go:96 +0x3c
github.com/prometheus/node_exporter/collector.(*cpuCollector).Update(0xc00005d180, 0xc0000de180, 0xc311a0, 0x0)
	/go/src/github.com/prometheus/node_exporter/collector/cpu_linux.go:81 +0xdc
github.com/prometheus/node_exporter/collector.execute(0x6e60b2, 0x3, 0x7cb140, 0xc00005d180, 0xc0000de180)
	/go/src/github.com/prometheus/node_exporter/collector/collector.go:127 +0x68
github.com/prometheus/node_exporter/collector.NodeCollector.Collect.func1(0xc0000de180, 0xc0000d82f0, 0x6e60b2, 0x3, 0x7cb140, 0xc00005d180)
	/go/src/github.com/prometheus/node_exporter/collector/collector.go:118 +0x48
created by github.com/prometheus/node_exporter/collector.NodeCollector.Collect
	/go/src/github.com/prometheus/node_exporter/collector/collector.go:117 +0xe2

This is due to format of /proc/cpuinfo being significantly different from the x86 version. Here's an example which causes the panic:

# cat /proc/cpuinfo
vendor_id       : IBM/S390
# processors    : 2
bogomips per cpu: 11061.00
features	: esan3 zarch stfle msa ldisp eimm dfp etf3eh highgprs 
processor 0: version = FF,  identification = 32C5C2,  machine = 2097
processor 1: version = FF,  identification = 32C5C2,  machine = 2097