If all the inputs are given as assembly files then we can skip rewriting
symbols for the first file. If this file is bcm.s (i.e. the large
compiler output), this can save a few seconds of build time.
Change-Id: I4e4ea114acb86cd93e831b23b58f8c3401bc711c
Reviewed-on: https://boringssl-review.googlesource.com/15149
Reviewed-by: Adam Langley <agl@google.com>