build: give CPPFLAGS some love, and make FPU  handling smarter for non-vanirified device trees

"FPU"? more like "FSU" - BEEYAWWW

ps1: firsties
ps2: handle determined cflags in TARGET_linux-arm, like -mcpu, etc. are handled
ps3: remove a $(warning ...) added for debugging
     use target_arch_variant_cflags for TARGET_GLOBAL_CPPFLAGS too (has -mcpu, -march, in addition to fpu stuff)
     remove stuff related to arch_variant_cppflags, which was just "arch_variant_cflags with less hotsauce"

Change-Id: Ic8dc75af2c18375dcf69046d48d9ff84dbffb7f0