[gdb/testsuite] Add KFAILs for gdb.ada FAILs with gcc-11

With gcc-11 we run into:
...
(gdb) print pa_ptr.all^M
That operation is not available on integers of more than 8 bytes.^M
(gdb) KFAIL: gdb.ada/arrayptr.exp: scenario=all: print pa_ptr.all (PRMS: gdb/20991)
...

This is due to PR exp/20991 - "__int128 type support".  Mark this and similar
FAILs as KFAIL.

Also mark this FAIL:
....
(gdb) print pa_ptr(3)^M
cannot subscript or call something of type `foo__packed_array_ptr'^M
(gdb) FAIL: gdb.ada/arrayptr.exp: scenario=minimal: print pa_ptr(3)
...
as a KFAIL for PR ada/28115 - "Support packed array encoded as
DW_TAG_subrange_type".

Tested on x86_64-linux, with gcc-10 and gcc-11.

gdb/testsuite/ChangeLog:

2021-07-21  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/arrayptr.exp: Add KFAILs for PR20991 and PR28115.
	* gdb.ada/exprs.exp: Add KFAILs for PR20991.
	* gdb.ada/packed_array_assign.exp: Same.

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