Fix internal uses of _PATH_BSHELL.

We regressed on this recently: code under the upstream-* directories has
_PATH_BSHELL defined as a call to __bionic_get_shell_path(). In our own
code, we may as well just call it directly.

Bug: https://issuetracker.google.com/129030706
Test: ran tests
Change-Id: Ic2423f521272be95e67f94771772fe8072636ef0