Misc fixes for configure

[matwey]

Currently,
./configure at armv6 machines ends up with

configure: error: unknown CPU type: armv6l

./configure at armv7 machines silently produces build for armv6 (compatible, but suboptimal)

configure: CFG_BUILD            := arm-unknown-linux-gnueabihf

[rust-highfive]

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[alexcrichton]

Thanks for the PR! Right now rustup has a similar block but also has this:

[alexcrichton]

Er, hit comment too soon, but rustup also has this:

# Detect armv7 but without the CPU features Rust needs in that build,
# and fall back to arm.
# See https://github.com/rust-lang-nursery/rustup.rs/issues/587.
if [ $_ostype = "unknown-linux-gnueabihf" -a $_cputype = armv7 ]; then
    if ensure grep '^Features' /proc/cpuinfo | grep -q -v neon; then
        # At least one processor does not have NEON.
        local _cputype=arm
    fi
fi

I wonder if that also needs to be included here? Could you also elaborate on the purpose of the PR in the description as well?

cc @brson

[matwey]

@alexcrichton I am not sure that your code snippet is necessary.

[alexcrichton]

Ah I erroneously thought that the armv7 Linux target enabled NEON, which we needed to specifically check for lack of support and go back to arm. Due to a recent PR, though, we no longer assume NEON on armv7, so this is fine.

@bors: r+

[bors]

📌 Commit 9b81f3c has been approved by alexcrichton