Don't build bionic-unit-tests-static with libcrt_builtins

libcrt_builtins does not work with libm, which is statically linked in
bionic-unit-tests-static. Opt it out of libcrt_builtins for now.

Bug: 119207651
Bug: 117167374
Test: run bionic-unit-test-static
Change-Id: I20067c9841beb9de2b73342847aaa945dbadbff8