enable parallel codegen by default

Enable parallel codegen (2 units) by default when --opt-level is 0 or 1.  This gives a minor speedup on large crates (~10%), with only a tiny slowdown (~2%) for small ones (which usually build in under a second regardless).  The current default (no parallelization) is used when the user requests optimization (--opt-level 2 or 3), and when the user has enabled LTO (which is incompatible with parallel codegen).

This commit also changes the rust build system to use parallel codegen when appropriate.  This means codegen-units=4 for stage0 always, and also for stage1 and stage2 when configured with --release-channel=source.  (Other release channel settings use codegen-units=1 for stage1 and stage2, to get maximum performance for release binaries.)  The build system also sets codegen-units=1 for compiletest tests (compiletest does its own parallelization) and uses the same setting as stage2 for crate tests.  The overall result (for release-channel=source) is 35% reduction in `make all` time, and 25% reduction in `make check` time.

r? @alexcrichton