libgcc_s: Add a linker script to link to libgcc

When using outline atomics on arm64 the compiler will create a call to
a function that performs the atomic operation. This allows us to use
the fastest operation depending on the hardware.

As these functions are implemented in libgcc create a linker script
so libraries that link against libgcc_s will include libgcc to pull
them in.

Reviewed by: imp, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45268