[ARMv7]: bad instruction br r2

[staticfloat]

The recently-resurrected ARMv7 buildbot is failing with the following error while compiling src/task.c:

{standard input}: Assembler messages:
{standard input}:133: Error: bad instruction `br r2'

Example error log: https://build.julialang.org/#/builders/5/builds/6/steps/4/logs/stdio

[yuyichao]

https://github.com/JuliaLang/julia/blame/5f2759d85eeeb9c68e35e5e84b10715a49f4449d/src/task.c#L730 needs to be bx. Introduced in #13099. I don't know why it was changed from b start_task though, since the latter should be one instruction shorter...

[staticfloat]

I've changed it from br to bx on this branch, but reading the ARM docs on b vs. bx makes me think that you are correct; we should be able to just use b instead of bx here, since I don't think we should ever be in thumb mode. @vtjnash do you have any insight to share?

[vtjnash]

I thought it was supposed to be common with AARCH64, I guess not, and should just use b.