Allow any build-id length between 4 and 20 bytes inclusive

Build-id support is being added to lld and by default it may produce a
64-bit build-id.

Prior to this change lldb would reject such a build-id. However, it then
falls back to a 4-byte crc32, which is a poorer quality identifier.

Differential Revision:	http://reviews.llvm.org/D18096

llvm-svn: 263432