gdb: Remove some C compiler support leftovers

gdb: Remove some C compiler support leftovers

Remove some __cplusplus checks, inline EXPORTED_CONST, and update some comments.

gdb/ChangeLog:
2016-10-06  Pedro Alves  <palves@redhat.com>

	* cp-valprint.c (vtbl_ptr_name): Write "extern const" instead of
	EXPORTED_CONST.
	* stub-termcap.c: Remove __cplusplus checks.
	* common/common-defs.h [!__cplusplus] (EXTERN_C, EXTERN_C_PUSH,
	EXTERN_C_POP): Delete.
	* common/common-exceptions.h (GDB_XCPT_SJMP): Update comments.
	(GDB_XCPT) [!__cplusplus]: Delete.
	(throw_exception, throw_exception_sjlj): Update comments.
	* guile/guile-internal.h (as_a_scm_t_subr) [!__cplusplus]: Delete.
	* guile/guile.c (extension_language_guile): Write "extern const"
	instead of EXPORTED_CONST.
	* features/feature_to_c.sh: Don't emit !__cplusplus code.  Write
	"extern const" instead of EXPORTED_CONST.