runtime: fix GOARM<7 build

Update issue 8690

If liblink determines that the host doesn't support TLS it replaces the MRC call with a BL runtime.tls_read_fallback. The problem is save_g doesn't expect anyone to make any BL calls and hasn't setup its own link register properly so when runtime.tls_read_fallback returns the LR points to save_g, not save_g's caller so the RET at the end of the function turns into an infinite loop.

This fix is only a proof of concept, I think the real fix should go into liblink as its MRC substitution is not as transparent as expected.

LGTM=rsc
R=rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/143050043