Add logic that will permit us to build user libraries with different CFLAGS than kernel code.  This is needed because we need the -fno-common option when building ELF code to prevent SHN_COMMON relocations

SAM3X/Arduino Due: Fix typo in sam3x_periphclks.h; add SCLK definitions to board.h header file.  From Fabien Comte

SAM3X/Arduino Due: Fix typo in sam3x_periphclks.h; add SCLK definitions to board.h header file.  From Fabien Comte

SAMA5D4-EK: In kernel build with address environment, need logic to map user virtual addresses to physical addresses, and vice versa

Fix some conditional compilation in kernel build mode.  And, while we are touching this logic, make g_default_pthread_attr const.