Add a -nostdlib specifically for the check to ensure that it builds
successfully.
The -nostdlib isn't needed normally, because linking against the pico
SDK will add in definitions for the previously-missing function
* Add missing DREQ_s
* store actual clock frequency in clock_configure (fixes#368)
* use dma DREQ values defined in dreqs/dma.h
* Fix hw_is_claimed, and add xxx_is_claimed APIs
* Add some PIO irq helper methods
* Add DMA channel IRQ status getter and clear methods
* Implement the correct PIO IRQ status/clear methods (good to have methods here as the h/w interrupt registers are super confusing)
* fix pico_multicore dependencies
* add missing wrapper func __aeabi_f2d
* Further DMA/PIO IRQ API cleanup (and review fixes)
* add PICO_INT64_OPS_IN_RAM flag
* Change some cmake output to DEBUG level
Make SDK build more consistent with other libraries (use an INTERFACE marker library for inclusion tests)
Add PICO_SDK_PRE_LIST_FILES, PICO_SDK_POST_LIST_FILES build vars
* fix typo
* remove leftover debugging message
* pico_stdio_usb: be more explicit about includes, fix warning (#257)
* pico_base: NDEBUG backwards for absolute_time_t (#255)
* pico_util: missing extern C in queue.h (#249)
* build: remove -march which was masking -mcpu, now SVC available (#253)
