Remove use of GOTPCREL in asm.

Now that symbol visibility is done correctly, this shouldn't be needed.

Change-Id: I608beed1de63c1309358ff17dd28e3191e87dbd4
Reviewed-on: https://boringssl-review.googlesource.com/1810
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>