xbps-src: introduce build profiles and use it by default.

common/build-profiles contains the compilation flags used in native
builds, and are defined per architecture:

	- x86_64 (glibc)
	- x86_64-musl (musl)

If XBPS_CFLAGS or XBPS_CXXFLAGS are set in `etc/conf`, the settings
from the build profile will be overrided.