gdb: introduce new function create_function_type

gdb: introduce new function create_function_type

While working on new Python API to create new function types I realized
that there's no easy way to create a new function type and control where
it is going to be allocated (whether in gdbarch's obstack or objfile's).
Functions lookup_function_type and lookup_function_type_with_arguments
always allocate at the same obstack as its return type.

This is not sufficient for the new Python API - the user may use any
type it can get hold of. For example, one may want to create a function
returning arch-owned type and taking one argument of objfile-owned type.
In that case we need to allocate the new type on that very objfile's
obstack.

This commit introduces new function - create_function_type - that takes
type_allocator as first parameter, allowing caller to control the
allocation. Existing functions (lookup_function_type and
lookup_function_type_with_arguments) are reimplemented by means of new
create_function_type.

Approved-By: Tom Tromey <tom@tromey.com>