[gdb/testsuite] Use find_gnatmake instead of gdb_find_gnatmake

[gdb/testsuite] Use find_gnatmake instead of gdb_find_gnatmake

On SLE-11, with an older dejagnu version, I ran into:
...
Running gdb.ada/mi_prot.exp ...
UNRESOLVED: gdb.ada/mi_prot.exp: \
  testcase aborted due to invalid command name: gdb_find_gnatmake
ERROR: tcl error sourcing gdb.ada/mi_prot.exp.
ERROR: invalid command name "gdb_find_gnatmake"
    while executing
"::gdb_tcl_unknown gdb_find_gnatmake"
    ("uplevel" body line 1)
    invoked from within
"uplevel 1 ::gdb_tcl_unknown $args"
    (procedure "::unknown" line 5)
    invoked from within
"gdb_find_gnatmake"
    (procedure "gnatmake_version_at_least" line 2)
    invoked from within
...

Proc gdb_find_gnatmake is actually a backup for find_gnatmake:
...
if {[info procs find_gnatmake] == ""} {
    rename gdb_find_gnatmake find_gnatmake
...
so gnatmake_version_at_least should use find_gnatmake instead.

For a recent dejagnu with find_gnatmake, gdb_find_gnatmake is kept, and we
don't run into this error.

For an older dejagnu without find_gnatmake, gdb_find_gnatmake is renamed to
find_gnatmake, and we do run into the error.

It's confusing that we're using the gdb_ prefix for gdb_find_gnatmake, it
seems something legitimate to use.  Maybe we should use future_ or gdb_future_
prefix instead to make this more clear, but I've left that alone for now.

Fix this by:
- triggering the same error with a recent dejagnu by removing
  gdb_find_gnatmake unless used (and likewise for other procs in future.exp),
  and
- using find_gnatmake in gnatmake_version_at_least.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR testsuite/31647
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31647