Setting the linker script as one property (instead of as part of the
generic ldflags property) allows it to be overriden.
This is important for the SoftDevice on Nordic chips, because the
SoftDevice takes up a fixed part of the flash/RAM and the application
must be flashed at a different position. With this linkerscript option,
it is possible to create (for example) a pca10040-s132v6 that overrides
the default linker script.
Move most of the logic of determining which compiler configuration to
use (such as GOOS/GOARCH, build tags, whether to include debug symbols,
panic strategy, etc.) into the compileopts package. This makes it a
single source of truth for anything related to compiler configuration.
It has a few advantages:
* The compile configuration is independent of the compiler package.
This makes it possible to move optimization passes out of the
compiler, as they don't rely on compiler.Config anymore.
* There is only one place to look if an incorrect compile option is
used.
* The compileopts provides some resistance against unintentionally
picking the wrong option, such as with c.selectGC() vs c.GC() in the
compiler.
* It is now a lot easier to change compile options, as most options
are getters now.
