[gomp4] Fix an ICE involving OpenACC routines inside broken fortran functions.

[Cesar Philippidis]

I encountered an ICE when the fortran FE tries to parse an OpenACC
routine directive when the containing function has a syntax error. E.g.

  integer function f1

is missing an argument list, so the fortran FE will not create a
function symbol for f1. Consequently, the OpenACC routine parser cannot
register the routine directive with any function, so it hit a
gcc_unreachable.

This patch replaces that gcc_unreachable with a match_error. While the
syntax of the routine directive may be correct, it is still an error to
have a dangling acc routine directive not associated with a function.

I've applied this patch to gomp-4_0-branch.

Cesar
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