[gdb/testsuite] Require gnatmake 8 for gdb.ada/rename_subscript_param.exp

[gdb/testsuite] Require gnatmake 8 for gdb.ada/rename_subscript_param.exp

When running gdb.ada/rename_subscript_param.exp with gnatmake 7.4.1, we get:
...
FAIL: gdb.ada/rename_subscript_param.exp: print rename_subscript_param_b \
  before changing its value
FAIL: gdb.ada/rename_subscript_param.exp: print rename_subscript_param_b \
  after changing its value
...

The commit last touching the test-case (afcfda091e) states:
...
    The test still fails with old compilers that do not properly
    generate debug info for this renaming:
...

Fix this by requiring at least gnatmake 8 for the test-case.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-09-14  Tom de Vries  <tdevries@suse.de>

	PR teststuite/24599
	* gdb.ada/rename_subscript_param.exp: Require gnatmake 8.
	* lib/ada.exp (gnatmake_version_at_least): New proc.