allocator_features uses buildflag_header to generate a header at
build time. In host/target builds, both the #host and #target
targets have a ninja rule for the same path, which is bad.
This CL makes allocator_features follow the same pattern of
base_debugging_flags (target-only, everything else explicitly refers
to the #target veriant)

I verified that the problem reproduces without this patch
by doing:
$ build/gyp_chromium -DOS=android -Duse_experimental_allocator_shim=1
$ ninja -w dupbuild=err -n -C out_android/Release/ all
ninja: warning: multiple rules generate gen/base/allocator/features.h. builds involving this target will not be correct;

And this CL fixes it.

BUG=593695

Review URL: https://codereview.chromium.org/1794943006

Cr-Commit-Position: refs/heads/master@{#381289}