[lldb][test] Prefer gmake to make and warn for potentially non-GNU make (#119573)

System make on FreeBSD is missing some GNU make features so out of the
box you get a lot of:
```
make: "<...>/Makefile.rules" line 569: Invalid line type
```

To solve this, you can install gmake which is a port of GNU make.
However because we prefer 'make', gmake won't be used unless you set
LLDB_TEST_MAKE.

To fix that, prefer 'gmake'. Also check (as best we can) that the make
we found is GNU make. This won't be perfect but it's better than the
cryptic error shown above.
```
-- Found make: /usr/bin/make
CMake Warning at /home/ec2-user/llvm-project/lldb/test/API/CMakeLists.txt:63 (message):
  'make' tool /usr/bin/make may not be GNU make compatible.  Some tests may
  fail to build.  Provide a GNU compatible 'make' tool by setting
  LLDB_TEST_MAKE.
```
When a make isn't found at all, the warning message will show the names
we tried:
```
-- Did not find one of: gmake make
CMake Warning at /home/ec2-user/llvm-project/lldb/test/API/CMakeLists.txt:69 (message):
  Many LLDB API tests require a 'make' tool.  Please provide it in Path or
  pass via LLDB_TEST_MAKE.
```

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