Update lang/gcc and hence the default version of GCC in the Ports

Collection (requested by USE_GCC=yes and various USES=compiler
invocations) from GCC 4.9.4 to GCC 5.4.

files/patch-arm-support and files/patch-gcc_system.h have become
obsolete.  New patches files/patch-arm-unwind-cxx-support and
files/patch-libc++ help support arm targets and new libc++ in base.

ONLY_FOR_ARCHS now also includes arm.

A new option GRAPHITE_DESC, off by default for now, adds support for
Graphite loop optimizations.

Finally, conflicts with other lang/gcc* ports are adjusted suitably.

In terms of changes for users, this upgrade brings the following:

The default mode for C is now -std=gnu11 instead of -std=gnu89.
New warning options -Wc90-c99-compat and -Wc99-c11-compat may
prove useful on that front.

The C++ front end now has full C++14 language support including
C++14 variable templates, C++14 aggregates with non-static data
member initializers, C++14 extended constexpr, and more.
The Standard C++ Library (libstdc++) has full C++11 support and
experimental full C++14 support.  It uses a new ABI by default.

There have been significant improvements to inter-procedural optimizations
and link-time optimization such as One Definition Rule based merging of C++
types as well as register allocation.

OpenMP 4.0 specification offloading features are now supported by the C,
C++, and Fortran compilers.  Cilk Plus, an extension to the C and C++
languages to support data and task parallelism, has been added as well.

New warning options -Wswitch-bool, -Wlogical-not-parentheses,
-Wbool-compare and -Wsizeof-array-argument may prove useful as
may new preprocessor directives __has_include, __has_include_next,
and __has_attribute.

GCC can now be built as a shared library for embedding in other processes
(such as interpreters), suitable for Just-In-Time compilation to machine
code.  This provides a C API and a C++ wrapper API.

Many code generation improvements for AArch64, ARM, support for
AVX-512{BW,DQ,VL,IFMA,VBMI} and Intel MPX on x86-64, and generally
improvements on many targets.

The Local Register Allocator (LRA) now contains a rematerialization
subpass and is able to reuse the PIC hard register on x86/x86-64 to
improve performance of position independent code.

https://gcc.gnu.org/gcc-5/changes.html has a more extensive set of
changes and https://gcc.gnu.org/gcc-5/porting_to.html has a solid
overview of issue you may encountering porting to this new version.

PR:             216707, 218125
Tested by:      antoine (-exp runs)
Supported by:   jbeich, tcberner, and others

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