GCC has two runtime libraries: The static library libgcc.a (-lgcc) and
the shared library libgcc_s.so (-lgcc_s). Both implement many of the
same functions but they also each have their unique functions. When
GCC links programs and libraries there are three possibilities:

1. gcc -static-libgcc or gcc -static: -lgcc => Just use libgcc.a.

2. gcc -shared-libgcc: -lgcc_s -lgcc => Link with libgcc_s first, so libgcc.a is only used for its unique functions.

3. gcc: -lgcc -Wl,--as-needed -lgcc_s -Wl,--no-as-needed => Link with libgcc.a first so libgcc_s is only used for its unique functions (_Unwind_* functions).

Approach 3 is the default for gcc and it's also what clang and clang++ use;
approach 2 is the default for gfortran, g++ and probably other front ends.

This patch makes 3 the default for gfortran. It significantly reduces
the use of libgcc_s. The _Unwind_* functions are also available in the
old base system libgcc_s which means this reduces the need for
-rpath /usr/local/lib/gccN in ports that depend on libraries built with
gfortran. Consider a dependency tree like this:

prog -> libA -> libgcc_s (old base system libgcc_s is fine)
     -> libB -> libgcc_s (libB built with gfortran, needs new libgcc_s)

Here prog needs to be linked with -rpath /usr/local/lib/gccN even if it's
a normal C program compiled with clang. Without -rpath it will fail to
start because it loads old libgcc_s first as a dependency of libA and then
it fails to load libB. With this patch libB works with old base system
libgcc_s or may not need libgcc_s at all, so prog does not need to be
linked with -rpath.

PR: 208120
Submitted by: tijl
MFH: 2019Q2 (important user visible improvement)