Fix b with zsh (again), remove shwordsplit use.

Context: zsh doesn't do string splitting like bash, so it passes the
post-processed bazel args to bazel as a single arg. So this CL adds a
conditional to split explicitly with zsh using the {=<arg>} flag
syntax.

Reference: https://zsh.sourceforge.io/FAQ/zshfaq03.html (for shwordsplit)
Reference: https://zsh.sourceforge.io/Guide/zshguide05.html (Section 5.4.4)

For bash, this uses array[@] to split the list into separate
shell words.

Test: zsh; source build/envsetup.sh && b test
//system/logging/logd:logd-unit-tests --config=linux_x86_64
Test: bash; source build/envsetup.sh && b test
//system/logging/logd:logd-unit-tests --config=linux_x86_64

Change-Id: Id92462661bbe1799a6de8dd3393799d91e3044e8