perf parse-events: Fix big-endian 'overwrite' by writing correct union member

The "Read backward ring buffer" test crashes on big-endian (e.g. s390x)
due to a NULL dereference when the backward mmap path isn't enabled.

Reproducer:
# ./perf test -F 'Read backward ring buffer'
Segmentation fault (core dumped)
# uname -m
s390x
#

Root cause:
get_config_terms() stores into evsel_config_term::val.val (u64) while later
code reads boolean fields such as evsel_config_term::val.overwrite.
On big-endian the 1-byte boolean is left-aligned, so writing
evsel_config_term::val.val = 1 is read back as
evsel_config_term::val.overwrite = 0,
leaving backward mmap disabled and a NULL map being used.

Store values in the union member that matches the term type, e.g.:
/* for OVERWRITE */
new_term->val.overwrite = 1; /* not new_term->val.val = 1 */
to fix this. Improve add_config_term() and add two more parameters for
string and value. Function add_config_term() now creates a complete node
element of type evsel_config_term and handles all evsel_config_term::val
union members.

Impact:
Enables backward mmap on big-endian and prevents the crash.
No change on little-endian.

Output after:
# ./perf test -Fv 44
--- start ---
Using CPUID IBM,9175,705,ME1,3.8,002f
mmap size 1052672B
mmap size 8192B
---- end ----
44: Read backward ring buffer : Ok
#

Fixes: 159ca97cd97ce8cc ("perf parse-events: Refactor get_config_terms() to remove macros")
Reviewed-by: James Clark <james.clark@linaro.org>
Reviewed-by: Jan Polensky <japo@linux.ibm.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>