arm: add macro for defining GPRs with aliases

On ARM, GPRs are often known by two names. E.g. the stack pointer is
both 'sp' and 'r13', the first parameter to a function 'a1' and 'r0',
etc. This macro allows defining them in a data structure, e.g. in the
ESF,  without having to create a union, use the correct type, etc: it is
less error-prone and makes for shorter code.

	sys_define_gpr_with_alias(name1, name2)

Change-Id: Ie4a6caa1ac23f26be4f7f0e05e9265f2655062cc
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>