tsweb/varz: add goamd64_capable and goamd64_compiled default metrics

To decide whether we can bump GOAMD64 for our production binaries, we
first need to know what x86-64 microarchitecture levels the fleet's
CPUs actually support. Add two gauges to the default metric set on
linux/amd64: goamd64_capable, the maximum GOAMD64 level the host CPU
supports per /proc/cpuinfo, and goamd64_compiled, the level the
running binary was built with per its build info. Both are computed
once at init and report 0 on error.

The per-level feature lists match the checks in Go's
runtime/asm_amd64.s.

Updates tailscale/corp#47099

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I3b4e5431ae1501f0b7d584c12d9af805e9ab6125