[gn build] Make 'compiler-rt' depend on include dir

That way, the headers in llvm/utils/gn/secondary/compiler-rt/include
are copied when running `ninja compiler-rt`. (Previously, they were
only copied when running `check-hwasan` or when building the
compiler-rt/include target.)

(Since they should be copied only once, depend on the target in the
host toolchain. I think default_toolchain should work just as well,
it just needs to be a single fixed toolchain. check-hwasan depends
through host_toolchain, so let's use that here too.)

Prevents errors like

    testing/fuzzed_data_provider.h:8:10: fatal error: 'fuzzer/FuzzedDataProvider.h' file not found

when building with locally-built clang. (For now, you still have to
explicitly build the 'compiler-rt' target. Maybe we should make the
clang target depend on that in the GN build?)

Differential Revision: https://reviews.llvm.org/D112238