cmd/5l: use BLX for BL (Rx).
Shenghou Ma authored
Fixes #5111.
Update #4718
This CL makes BL (Rx) to use BLX Rx instead of:
MOV LR, PC
MOV PC, Rx

R=cshapiro, rsc
CC=dave, gobot, golang-dev
https://golang.org/cl/9669045
35e1deae