runtime: Fix build on older ARM
runtime: Fix build on older ARM

        The ARM external linking CL used BLX instructions in gcc assembler. Replace with BL to retain support on older ARM processors.

R=rsc
CC=golang-dev
https://codereview.appspot.com/12938043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go