Set default visibility on art_sigsegv_fault.

libart.so is built with flag -fvisibility=protected
which complicates setting breakpoints on art_sigsegv_fault.
According to the GCC manual "Protected visibility indicates
that the symbol will be placed in the dynamic symbol table,
but that references within the defining module will bind
to the local symbol.". This implies that a breakpoint on
a protected symbol will not be hit.

Without this change gdbclient with ART debugging enabled
doesn't break at art_sigsegv_fault as expected.

Change-Id: Ib73344e710966eddf55a6facb29558504a511f00