loader, unicode/utf8: fix hiBits overflow on 16-bit AVR targets

Go 1.26 added SWAR optimizations to unicode/utf8 that use:

  const ptrSize = 4 << (^uintptr(0) >> 63)
  const hiBits = 0x8080808080808080 >> (64 - 8*ptrSize)

This formula only distinguishes 32-bit and 64-bit architectures.
On AVR (16-bit uintptr), ptrSize computes as 4, and hiBits becomes
0x80808080 (2155905152) which overflows the 16-bit uintptr type.

Fix by providing a patched unicode/utf8 overlay for Go 1.26+ that
uses a ptrSize formula handling all three sizes (16/32/64-bit):

  const ptrSize = 1 << (^uintptr(0)>>15&1 + ^uintptr(0)>>31&1 + ^uintptr(0)>>63&1)

This evaluates to 2 on AVR, 4 on 32-bit, and 8 on 64-bit. The
word() helper also gains a ptrSize==2 case for 16-bit loads.

loader/goroot.go

@@ -273,6 +273,16 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 		paths["internal/syscall/"] = true
 		paths["internal/syscall/unix/"] = false
 	}
+
+	if goMinor >= 26 {
+		// Go 1.26 added SWAR optimizations to unicode/utf8 that use
+		// constants assuming at least 32-bit uintptr. TinyGo supports
+		// 16-bit targets (AVR) where these constants overflow, so we
+		// provide a patched version.
+		paths["unicode/"] = true
+		paths["unicode/utf8/"] = false
+	}
+
 	return paths
 }