CL 56120043 fixed and cleaned up TLS on ARM after introducing liblink, but
left flag_shared broken. This CL restores the (unsupported) flag_shared
behaviour by simply rewriting access to $runtime.tlsgm(SB) with
runtime.tlsgm(SB), to compensate for the extra indirection when going from
the R_ARM_TLS_LE32 relocation to the R_ARM_TLS_IE32 relocation.

Also, remove unnecessary symbol lookup left after 56120043.

LGTM=iant
R=iant, rsc
CC=golang-codereviews
https://golang.org/cl/57000043