[QUESTION] Warnings when link with LTO

[pkarashchenko]

Description

I was trying to build code with GCC and LTO enabled. I observe link time warnings

lto-wrapper: warning: Extra option to '-Xassembler': -mthumb, dropping all '-Xassembler' and '-Wa' options.
lto-wrapper: warning: using serial compilation of 2 LTRANS jobs
lto-wrapper: note: see the '-flto' option documentation for more information

I see that it is because of ARCHOPTIMIZATION += -Wa,-mthumb and ARCHOPTIMIZATION += -Wa,-mimplicit-it=always that were added by 9ab97df0a57 and 77aede7c87b

The 9ab97df0a57 has description from GCC 4.5.x and in later GCC versions I do not see description that requires explicit passing of -mthumb with -Wa option to assembler. Do we still target GCC 4.5.x support?

For 77aede7c87b I would like to reproduce an error if possible locally and check. I see that commit message references to ARM Clang, but -Wa,-mimplicit-it=always is passed without checking for ifeq ($(CONFIG_ARM_TOOLCHAIN_ARMCLANG),y), so I would to better understand the issue and a way to fix it.

Additionally to above I do not see that we use -mthumb-interwork option and I would like to evaluate if we can fix any of above issues with Thumb interworking.

Verification

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[pkarashchenko]

@anchao and @xiaoxiang781216 for visibility