Remove BUILDING_LIBART macro.

It has been used only to prevent including the libart header
`runtime_asm_entrypoints.h` when compiling other targets.
Instead make symbols in that header hidden so that using
them from other targets shall yield a link time error. (This
is somewhat pointless because the header defines only inline
functions but the actual entrypoint symbols defined in
assembly are already hidden anyway.)

This change adds the symbol visibility macros `HIDDEN` and
`EXPORT` which were originally included in the reverted CL
    https://android-review.googlesource.com/1137699 .

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 186902856
Change-Id: Ia9fed6b3f49d7356888d702f4ad3bb48e5884793
5 files changed