all: add support for ThinLTO

ThinLTO optimizes across LLVM modules at link time. This means that
optimizations (such as inlining and const-propagation) are possible
between C and Go. This makes this change especially useful for CGo, but
not just for CGo. By doing some optimizations at link time, the linker
can discard some unused functions and this leads to a size reduction on
average. It does increase code size in some cases, but that's true for
most optimizations.

I've excluded a number of targets for now (wasm, avr, xtensa, windows,
macos). They can probably be supported with some more work, but that
should be done in separate PRs.

Overall, this change results in an average 3.24% size reduction over all
the tinygo.org/x/drivers smoke tests.

TODO: this commit runs part of the pass pipeline twice. We should set
the PrepareForThinLTO flag in the PassManagerBuilder for even further
reduced code size (0.7%) and improved compilation speed.

compileopts/config.go

@@ -176,6 +176,34 @@ func (c *Config) AutomaticStackSize() bool {
 	return false
 }
 
+// UseThinLTO returns whether ThinLTO should be used for the given target. Some
+// targets (such as wasm) are not yet supported.
+// We should try and remove as many exceptions as possible in the future, so
+// that this optimization can be applied in more places.
+func (c *Config) UseThinLTO() bool {
+	parts := strings.Split(c.Triple(), "-")
+	if parts[0] == "wasm32" {
+		// wasm-ld doesn't seem to support ThinLTO yet.
+		return false
+	}
+	if parts[0] == "avr" || parts[0] == "xtensa" {
+		// These use external (GNU) linkers which might perhaps support ThinLTO
+		// through a plugin, but it's too much hassle to set up.
+		return false
+	}
+	if len(parts) >= 2 && strings.HasPrefix(parts[2], "macos") {
+		// We use an external linker here at the moment.
+		return false
+	}
+	if len(parts) >= 2 && parts[2] == "windows" {
+		// Linker error (undefined runtime.trackedGlobalsBitmap) when linking
+		// for Windows. Disable it for now until that's figured out and fixed.
+		return false
+	}
+	// Other architectures support ThinLTO.
+	return true
+}
+
 // RP2040BootPatch returns whether the RP2040 boot patch should be applied that
 // calculates and patches in the checksum for the 2nd stage bootloader.
 func (c *Config) RP2040BootPatch() bool {