sim/igen: Fix linker error with -fno-common

sim/igen: Fix linker error with -fno-common

GCC 10 enables -fno-common by default.  This resulted in multiple
definition linker errors since a global variable was declared and
defined in a header file:

  ld: libsim.a(idecode.o):sim/v850/idecode.h:71: multiple definition of
  `idecode_issue'; libsim.a(irun.o):sim/v850/idecode.h:71: first defined
  here

  ld: libsim.a(engine.o):sim/v850/idecode.h:71: multiple definition of
  `idecode_issue'; libsim.a(irun.o):sim/v850/idecode.h:71: first defined
  here

  ld: libsim.a(support.o):sim/v850/idecode.h:71: multiple definition of
  `idecode_issue'; libsim.a(irun.o):sim/v850/idecode.h:71: first defined
  here

  ld: libsim.a(semantics.o):sim/v850/idecode.h:71: multiple definition
  of `idecode_issue'; libsim.a(irun.o):sim/v850/idecode.h:71: first
  defined here

sim/igen

	PR sim/26194

	* lf.h (lf_get_file_type): Declare.
	* lf.c (lf_get_file_type): Define.
	* gen-idecode.c (print_idecode_issue_function_header): Use
	lf_get_file_type() to issue an extern variable declaration in
	case of header files.