guile: fix make-value with pointer type

guile: fix make-value with pointer type

Calling the `make-value' procedure with an integer value and a pointer
type for the #:type argument triggers a failed assertion in
`get_unsigned_type_max', as that function doesn't consider pointers to
be an unsigned type. This commit fixes the issue by adding a separate
code path for pointers.

As previously suggested, range checking is done using a new helper
function in gdbtypes.

gdb/ChangeLog:

2021-07-30  George Barrett  <bob@bob131.so>

	* gdbtypes.h (get_pointer_type_max): Add declaration.
	* gdbtypes.c (get_pointer_type_max): Add definition for new
	helper function.
	* guile/scm-math.c (vlscm_convert_typed_number): Add code path
	for handling conversions to pointer types without failing an
	assert.

gdb/testsuite/ChangeLog:

2021-07-30  George Barrett  <bob@bob131.so>

	* gdb.guile/scm-math.exp (test_value_numeric_ops): Add test
	for creating pointers with make-value.
	(test_make_pointer_value, test_pointer_numeric_range): Add
	test procedures containing checks for integer-to-pointer
	validation.

Change-Id: I9994dd1c848840a3d995f745e6d72867732049f0