Fix --sysroot for cross-toolchain GCC packages

By default, GCC assumes that a cross compiler should not have any valid
initial include or library paths aside from /usr/local/lib/gcc/<mumble>/.
This means that one cannot use --sysroot to point to a system root for
another architecture and have GCC automatically look for headers in
${sysroot}/usr/include, etc. Currently we workaround this in FreeBSD's
build system with explicit -isystem, -B, and -L directives. However,
this is cumbersome compared to clang (where a bare --sysroot DTRT)
especially when using the compiler to build other software (such as test
programs, etc.).

One can override GCC's assumption and force it to assume that it should
honor --sysroot by setting the '--with-sysroot' option to force GCC to
assume a specific system root. By setting this to '/', this means that
the cross-compiler will attempt to use the host's headers by default
if --sysroot is not specified, but if --sysroot is specified then
it is fully honored including for include paths and library paths. With
this change I can now cross-compile both C and C++ binaries simply by
using --sysroot without the need for -isystem, -B, or -L directives. Note
that the base/gcc and devel/riscv64-gcc ports both use --with-sysroot='/'
already.

By default, GCC looks for headers in /usr/local/include (under the
sysroot) before /usr/include. To disable this and only look for headers
in /usr/include, patch gcc/Makefile.in to not define LOCAL_INCLUDE_DIR.

Once -nostdinc is no longer required, the headers installed along with the
compiler are now used for compiling worlds and test programs. The "fixed"
headers in include-fixed are generally not helpful and are also derived
from the host's headers which might not match the target --sysroot, so
just delete them entirely. Even the stub limits.h headers GCC ships when
using an empty build sysroot are not helpful and need to be removed.

On a related note, I also fixed the name of the C++ include directory
option to configure. By my reading it is 'with-gxx-include-dir' rather
than 'with-gcc-include-dir'.

Reviewed by: bapt, brooks, kan
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D14627