This replaces the target-selection options (such as "--enable-vfp3")
with a simpler, absolute "--arm-arch" option. This eliminates inferences
and avoids surprising behaviour in impossible situations (such as
"--enable-vfp3 --no-enable-armv7").

The available options are:
    --arm-arch=armv6          ARMv6 + VFPv2
    --arm-arch=armv7          ARMv7 + VFPv3-D32 + NEON
    --arm-arch=armv7+sudiv    ARMv7 + VFPv4-D32 + NEON + SUDIV
    --arm-arch=armv8          ARMv8 (+ all of the above)

For now, the default setting is "armv8", which results in behaviour very
similar to the existing defaults.

BUG=v8:5077

Review-Url: https://codereview.chromium.org/2223433002
Cr-Commit-Position: refs/heads/master@{#39004}