cmd/compile: make TestAssembly resilient to output ordering
To preserve reproducible builds, the text entries
during compilation will be sorted before being printed.
TestAssembly currently assumes that function init
comes after all user-defined functions.
Remove that assumption.
Instead of looking for "TEXT" to tell you where
a function ends--which may now yield lots of
non-function-code junk--look for a line beginning
with non-whitespace.