perf tool x86: Fix perf_env memory leak

In the Linux kernel, the following vulnerability has been resolved:

perf tool x86: Fix perf_env memory leak

Found by leak sanitizer:
```
==1632594==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 21 byte(s) in 1 object(s) allocated from:
#0 0x7f2953a7077b in __interceptor_strdup ../../../../src/libsanitizer/asan/asan_interceptors.cpp:439
#1 0x556701d6fbbf in perf_env__read_cpuid util/env.c:369
#2 0x556701d70589 in perf_env__cpuid util/env.c:465
#3 0x55670204bba2 in x86__is_amd_cpu arch/x86/util/env.c:14
#4 0x5567020487a2 in arch__post_evsel_config arch/x86/util/evsel.c:83
#5 0x556701d8f78b in evsel__config util/evsel.c:1366
#6 0x556701ef5872 in evlist__config util/record.c:108
#7 0x556701cd6bcd in test__PERF_RECORD tests/perf-record.c:112
#8 0x556701cacd07 in run_test tests/builtin-test.c:236
#9 0x556701cacfac in test_and_print tests/builtin-test.c:265
#10 0x556701cadddb in __cmd_test tests/builtin-test.c:402
#11 0x556701caf2aa in cmd_test tests/builtin-test.c:559
#12 0x556701d3b557 in run_builtin tools/perf/perf.c:323
#13 0x556701d3bac8 in handle_internal_command tools/perf/perf.c:377
#14 0x556701d3be90 in run_argv tools/perf/perf.c:421
#15 0x556701d3c3f8 in main tools/perf/perf.c:537
#16 0x7f2952a46189 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58

SUMMARY: AddressSanitizer: 21 byte(s) leaked in 1 allocation(s).
```