ARCv2: entry: rewrite to enable use of double load/stores LDD/STD

[ Upstream commit a4880801a72ecc2dcdfa432f81a754f3e7438567 ]

 - the motivation was to be remove blatent copy-paste due to hasty support
   of CONFIG_ARC_IRQ_NO_AUTOSAVE support

 - but with refactoring we could use LDD/STD to greatly optimize the code

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Stable-dep-of: 92e2921eeafd ("ARC: define ASM_NL and __ALIGN(_STR) outside #ifdef __ASSEMBLY__ guard")
Signed-off-by: Sasha Levin <sashal@kernel.org>