targets/pca10040: support linking from within the compiler

This is kind of dirty with that huge list of linker params, but it works
and it produces smaller object files (probably because GCC is better
optimized for size).

targets/pca10040.json

@@ -1,4 +1,6 @@
 {
 	"llvm-target": "armv7m-none-eabi",
-	"build-tags": ["nrf", "nrf52", "nrf52832", "js", "wasm"]
+	"build-tags": ["nrf", "nrf52", "nrf52832", "js", "wasm"],
+	"linker": "arm-none-eabi-gcc",
+	"pre-link-args": ["-nostdlib", "-nostartfiles", "-mcpu=cortex-m4", "-mthumb", "-T", "arm.ld", "-Wl,--gc-sections", "-fno-exceptions", "-fno-unwind-tables", "-ffunction-sections", "-fdata-sections", "-Os", "-DNRF52832_XXAA", "-D__STARTUP_CLEAR_BSS", "-Ilib/CMSIS/CMSIS/Include", "lib/nrfx/mdk/gcc_startup_nrf51.S", "lib/nrfx/mdk/system_nrf52.c"]
 }