compiler: allow arbitrary label string for JSFFI exports

This commit allows arbitrary label string to appear in a foreign
export declaration, as long as the calling convention is javascript.
Well, doesn't make sense to enforce it's a C function symbol for a
JSFFI declaration anyway, and it gets in the way of implementing the
"sync" flavour of exports.

(cherry picked from commit a204df3a)
(cherry picked from commit 18dd15dc)