arch: allow GCC target options to be optionally overwritten

arch: allow GCC target options to be optionally overwritten

The BR2_GCC_TARGET_* configuration variables are copied to
corresponding GCC_TARGET_* variables which may then be optionally
modified or overwritten by architecture specific makefiles.

All makefiles must use the new GCC_TARGET_* variables instead
of the BR2_GCC_TARGET_* versions.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
[Thomas: simplify include of arch/arch.mk]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>