fix(libhsakmt): Define _GNU_SOURCE before libhsakmt.h include (#8555)

## Summary

Move `#define _GNU_SOURCE` to the top of `hsakmtmodel.c`, before all
includes, and remove the redundant `#define __USE_GNU`. `_GNU_SOURCE`
must be defined before any system header is pulled in; `hsakmtmodel.h`,
`hsakmt_drm.h`, and `libhsakmt.h` all transitively include libc/drm
headers. Defining it after those includes breaks GNU extension
visibility (e.g. `secure_getenv`) and causes compile errors in strict or
sanitizer builds.

## JIRA ID

JIRA ID : ROCM-26861

## Test plan

Tested on **dayatsin-test-01** (Ubuntu 24.04, amdclang 23.0.0 from
`/opt/rocm`), including after removing `__USE_GNU` (`c651d3f278`):

- [x] Build `libhsakmt` / `rocr-runtime` with ASAN enabled
(`-DADDRESS_SANITIZER=ON -DTHEROCK_SANITIZER=ASAN`) — full build
succeeded (`libhsa-runtime64.so` produced)
- [x] Build with amdclang from TheRock/ROCm toolchain — Release `hsakmt`
target succeeded
- [x] Verify `hsakmtmodel.c` compiles without `_GNU_SOURCE` ordering
warnings/errors — no errors in either ASAN or Release builds

Author: erman-gurses

projects/rocr-runtime/libhsakmt/src/hsakmtmodel.c

@@ -23,13 +23,13 @@
  * DEALINGS IN THE SOFTWARE.
  */
 
+#define _GNU_SOURCE
+
 #include "hsakmt/hsakmtmodel.h"
 #include "hsakmt/hsakmt_drm.h"
 #include "libhsakmt.h"
 #include "hsakmt/hsakmttypes.h"
 #include "hsakmt/hsakmtmodeliface.h"
-#define _GNU_SOURCE
-#define __USE_GNU
 #include <assert.h>
 #include <errno.h>
 #include <inttypes.h>