Fix compile error for non tls code branch

Summary: New VASM code has a new template for how emitTLSLoad work
however there is no corresponding template for the non fast-tls branch so the compile is failing

Since there is currently only one call into the new version of emitTLSLoad from emitGCContext
this has the non fast tls branch use the previous call until the new code has working stuff for the non-fast tls branch as well

This is also a fix for github issue #3519

Reviewed By: @edwinsmith

Differential Revision: D1508584