Eventually we might want to start using the FPU, but the easy option
right now is to simply disable it everywhere. Previously, it depended on
whether Clang was built as part of TinyGo or it was an external binary.
By setting the floating point mode explicitly, such inconsistencies are
avoided.
This commit creates a new cortex-m4 target which can be the central
place for setting FPU-related settings across all Cortex-M4 chips.
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.
