Move libgcc to libcrt.builtins

libgcc is kept behind libcrt.builtins to provide unwinder symbols and
any other missing symbols.

libc and libm are excluded from the switch due to some symbols being
hidden in libcrt.builtins but not in libgcc. These will be addressed in
a separate CL.

Bug: 29275768
Test: checkbuild and bionic native tests on arm/arm64/x86/x86_64
Change-Id: Icc85bf88513e989a4b1547564405a22cf847db8a