tcmalloc: repair the build for non-rseq enabled platforms
tcmalloc: repair the build for non-rseq enabled platforms

When building for non-rseq enabled platforms, we would fail to build due to the
use of the `shift` as an integral value which no longer holds.  Explicitly
convert the value to an integral type according to the use.  This repairs the
build for RISC-V targets.

PiperOrigin-RevId: 422628738
Change-Id: Ia39e18a466ca6c0ba2c14e25ea8e6063aab76e31

google/tcmalloc,google/tcmalloc,google/tcmalloc