Fail-proof the inline directives for SMP atomic operations.

The various atomic operations are declared as inline in the header files
to cut the function call overhead. However, the plain inline keyword is
just a suggestion to the compiler which makes its own decision on whether
to inline them or not. Worst, if they are not inlined, the
-fvisibility-inlines-hidden compiler flag will render them as hidden
symbols in the object file. If they are picked up by the linker over the
ones supplied in libcutils.so the following warning will be reported:

warning: hidden symbol 'android_atomic_inc' in foo.o is referenced by
DSO bar.so

One way is to add __attribute__((visibility("default"))) to those inline
functions to suppress the linker warnings. A better way is to force
inlining as with this patch.

Change-Id: Ie4fcfdfaaf06f42d351619a0d89671a9df15ca2f