Bug#801303: libc++: FTBFS on arm64, needs llvm-3.7

[Edmund Grimley Evans]

Source: libc++
Version: 3.7.0-1

It failed to build on arm64:

https://buildd.debian.org/status/package.php?p=libc%2B%2B&suite=sid

The error was a segmentation fault in 'Greedy Register Allocator'.

There are similar reports elsewhere, such as bug #796343:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=796343

It seems that arm64 needs llvm-3.7.