[AVR,test] Change llc -march= to -mtriple=

Similar to 806761a7629df268c8aed49657aeccffa6bca449

-mtriple= specifies the full target triple while -march= merely sets the
architecture part of the defa

[AVR,test] Change llc -march= to -mtriple=

Similar to 806761a7629df268c8aed49657aeccffa6bca449

-mtriple= specifies the full target triple while -march= merely sets the
architecture part of the default target triple, leaving a target triple which
may not make sense.

Therefore, -march= is error-prone and not recommended for tests without a target
triple. The issue has been benign as we recognize avr-apple-darwin as ELF instead
of rejecting it outrightly.

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[AVR] Fix 16-bit LDDs with immediate overflows (#104923)

16-bit loads are expanded into a pair of 8-bit loads, so the maximum
offset of such 16-bit loads must be 62, not 63.