perf bpf: Use __builtin_preserve_field_info for GCC compatibility

When exploring building bpf_skel with GCC's BPF support, there was a
build failure because of bpf_core_field_exists vs the mem_hops bitfield:
```
In file included from util/bpf_skel/sample_filter.bpf.c:6:
util/bpf_skel/sample_filter.bpf.c: In function 'perf_get_sample':
tools/perf/libbpf/include/bpf/bpf_core_read.h:169:42: error: cannot take address of bit-field 'mem_hops'
169 | #define ___bpf_field_ref1(field) (&(field))
| ^
tools/perf/libbpf/include/bpf/bpf_helpers.h:222:29: note: in expansion of macro '___bpf_field_ref1'
222 | #define ___bpf_concat(a, b) a ## b
| ^
tools/perf/libbpf/include/bpf/bpf_helpers.h:225:29: note: in expansion of macro '___bpf_concat'
225 | #define ___bpf_apply(fn, n) ___bpf_concat(fn, n)
| ^~~~~~~~~~~~~
tools/perf/libbpf/include/bpf/bpf_core_read.h:173:9: note: in expansion of macro '___bpf_apply'
173 | ___bpf_apply(___bpf_field_ref, ___bpf_narg(args))(args)
| ^~~~~~~~~~~~
tools/perf/libbpf/include/bpf/bpf_core_read.h:188:39: note: in expansion of macro '___bpf_field_ref'
188 | __builtin_preserve_field_info(___bpf_field_ref(field), BPF_FIELD_EXISTS)
| ^~~~~~~~~~~~~~~~
util/bpf_skel/sample_filter.bpf.c:167:29: note: in expansion of macro 'bpf_core_field_exists'
167 | if (bpf_core_field_exists(data->mem_hops))
| ^~~~~~~~~~~~~~~~~~~~~
cc1: error: argument is not a field access
```

___bpf_field_ref1 was adapted for GCC in 12bbcf8e840f40b82b02981e96e0a5fbb0703ea9
but the trick added for compatibility in 3a8b8fc3174891c4c12f5766d82184a82d4b2e3e
isn't compatible with that as an address is used as an argument.

Workaround this by calling __builtin_preserve_field_info directly as the
bpf_core_field_exists macro does, but without the ___bpf_field_ref use.

Co-developed-by: Andrew Pinski <[e-mail]>
Signed-off-by: Andrew Pinski <[e-mail]>
Signed-off-by: Sam James <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Tested-by: Namhyung Kim <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Kan Liang <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://gcc.gnu.org/PR121420
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/bpf_skel/sample_filter.bpf.c

···
 		if (entry->part == 8) {
 			union perf_mem_data_src___new *data = (void *)&kctx->data->data_src;
 
-			if (bpf_core_field_exists(data->mem_hops))
+			if (__builtin_preserve_field_info(data->mem_hops, BPF_FIELD_EXISTS))
 				return data->mem_hops;
 
 			return 0;