[libc] Fix sort test failing on NVPTX

Summary:
This test uses too much stack and crashes, make the buffer `static` to
push it to `.bss`. This shouldn't change behavior because the tests are
all run single threaded.

libc/test/src/stdlib/SortingTest.h

@@ -298,8 +298,9 @@ class SortingTest : public LIBC_NAMESPACE::testing::Test {
     static_assert(ARRAY_LEN < 256); // so we can encode the values.
 
     // Minimum alignment to test implementation for bugs related to assuming
-    // incorrect association between alignment and element size.
-    alignas(1) uint8_t buf[BUF_SIZE];
+    // incorrect association between alignment and element size. The buffer is
+    // 'static' as otherwise it will exhaust the stack on the GPU targets.
+    alignas(1) static uint8_t buf[BUF_SIZE];
 
     // GCC still requires capturing the constant ARRAY_INITIAL_VALS in the
     // lambda hence, let's use & to implicitly capture all needed variables