Add -march=armv7-a parameter to armv7 android linker

[malbarbo]

Without this option, the linker fails to link any library that uses std::future. The error points some undefined references, like std::__future_base::_Result_base.

For example, it fails to link rustc because llvm 4.0 uses std::future.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

[rust-highfive]

r? @arielb1

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[arielb1]

r? @japaric you understand ARM better than I do

[arielb1]

Thanks for the PR @malbarbo. @japaric or someone else should be reviewing your PR shortly.

[malbarbo]

Here is a script to reproduce the problem. It is necessary to have ndk installed and the android gcc compilers on path.

[alexcrichton]

@bors: r+

Thanks @malbarbo!

[bors]

📌 Commit 9896483 has been approved by alexcrichton