[SH] [libstdc++] Workaround for PR 29366

[Oleg Endo]

Hi,

The attached patch is a temporary workaround for PR 29366.  We've been
having support for various atomics on SH for a while, but libstdc++'s
configury makes it impossible to use when doing a cross build (PR
53579).  Even if that worked, if I'm not mistaken,
config/cpu/sh/atomicity.h overrides all of that and replaces it with its
own SH4A atomics or a lock based implementation for everything non-SH4A.
The patch works OK on sh-elf, and as far as I can see there should be no
problems on sh*-linux configs, too.  I will commit the patch if there
are no objections in 48h.

Cheers,
Oleg

libstdc++-v4/ChangeLog:
	PR target/29366
	* config/cpu/sh/atomicity.h (__exchange_and_add, __atomic_add): 
	Remove SH4A inline asm and lock based implementations and use 
	the defaults from ext/atomicity.h.
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