Add a `build_type` variant, which allows building optimized compilers,
as well as target libraries (libstdc++ and friends).
The default is `build_type=RelWithDebInfo`, which corresponds to GCC's
default of -O2 -g.
When building with `+bootstrap %gcc`, also add Spack's arch specific
flags using the common denominator between host and new GCC.
It is done by creating a config/spack.mk file in def patch, that looks
as follows:
```
BOOT_CFLAGS := $(filter-out -O% -g%, $(BOOT_CFLAGS)) -O2 -g -march=znver2 -mtune=znver2
CFLAGS_FOR_TARGET := $(filter-out -O% -g%, $(CFLAGS_FOR_TARGET)) -O2 -g -march=znver2 -mtune=znver2
CXXFLAGS_FOR_TARGET := $(filter-out -O% -g%, $(CXXFLAGS_FOR_TARGET)) -O2 -g -march=znver2 -mtune=znver2
```
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