tests: benchmark: boot_time: Reading time stamps made arch agnostic

1. Changed _tsc_read() to k_cycles_get_32(). Thus reading the
time stamp will be agnostic of the architecutre used.
2. Changed the variable names from *_tsc to *_time_stamp.

JIRA: ZEP-1426

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>