[compiler-rt] Use asm .set only for Hexagon (#194160)
Two incompatible assembler syntaxes exist for symbol assignment:
```
sym = val -- accepted by most GNU assembler targets; rejected by
Hexagon, which interprets it as a mnemonic
.set sym, val -- accepted by Hexagon; rejected by Alpha, which
reserves .set for assembler mode flags
```
Switch all to `sym = val`, and opt out Hexagon to `.set sym`.
Fixes: dbb03f8f606e ("[compiler-rt] Replace assignment w/.set directive
(#107667)")
---------
Co-authored-by: Vitaly Buka <vitalybuka@google.com>
