[gdb/testsuite] Require gnatmake 11 for gdb.ada/verylong.exp

With test-case gdb.ada/verylong.exp and gnatmake 7.5.0 I run into:
...
compilation failed: gcc ... $src/gdb/testsuite/gdb.ada/verylong/prog.adb
prog.adb:16:11: warning: file name does not match unit name, should be "main.adb"
prog.adb:17:08: "Long_Long_Long_Integer" is undefined (more references follow)
gnatmake: "prog.adb" compilation error

FAIL: gdb.ada/verylong.exp: compilation prog.adb
...

AFAICT, support for Long_Long_Long_Integer was added in gcc 11.

Fix this by requiring gnatmake version 11 or higher in the test-case.

Tested on x86_64-linux.