Add target.linux_glibc and target.bionic

target.linux_glibc will apply to host builds with glibc, which is
identical to the current target.linux. In a future change, target.linux
will change to affect all targets using the Linux kernel (android,
linux_bionic, and linux_glibc).

target.bionic will apply to all OS variants using Bionic.

Bug: 31559095
Test: Add target.linux_glibc, target.bionic sections to an Android.bp, build
Test: m host
Change-Id: I677a67c22fba148fec264132311e355283f9d88d