Minimum android NDK version should be updated

[ejjeong]

On android NDK version before r9d, it only provided gdb 7.3.
And gdb 7.3 cannot understand DWARF4, which is generated by llvm by default.

Minimum NDK version should be updated like below (both for buildbot & wiki):

• ndk version > r10
• toolchain gcc version > gcc 4.8

For now, instead of upgrading the NDK version,
the default dwarf version will be lowered as DWARF2, like OSX.

[lahwran]

Was this fixed by ejjeong's commit?

[lahwran]

looks like it wasn't.

[ejjeong]

It's not fixed yet.

[steveklabnik]

Triage: it's unclear to me where the minimum ndk version is specified. So I can't tell if this has been fixed or not.

[lahwran]

I don't remember at this point

[alexcrichton]

Right now we actually somewhat guarantee API level 18 in the standard library (compatibility-wise), so we're probably not going to update this. CI we've got basically taken care of, so we should be good on that front.

[ejjeong]

FYI: https://github.com/rust-lang/rust-wiki-backup/blob/master/Doc-building-for-android.md
This old wiki had described that minimum ndk version is r9b.