BaseTools GCC: introduce GCC5 toolchain to support GCC v5.x in LTO mode

BaseTools GCC: introduce GCC5 toolchain to support GCC v5.x in LTO mode

This adds support for GCC 5.x in LTO mode for IA32, X64, ARM and
AARCH64. Due to the fact that the GCC project switched to a new
numbering scheme where the first digit is now incremented for every
major release, the new toolchain is simply called 'GCC5', and is
intended to support all GCC v5.x releases.

Since IA32 and X64 enable compiler optimizations (-Os) for both DEBUG
and RELEASE builds, LTO support is equally enabled for both targets.
On ARM and AARCH64, DEBUG builds are not optimized, and so the LTO
optimizations are only enabled for RELEASE.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>