all: shrink bdwgc library

By disabling some configuration options, the library becomes a lot
smaller. With `-no-debug`, binaries become ~25kB smaller on Linux, ~5kB
smaller on MacOS, and ~11kB smaller on Windows.

This isn't really important on Linux/macOS/Windows, but will become
important when we start using bdwgc on WebAssembly.

Author: aykevl

builder/bdwgc.go

@@ -30,6 +30,10 @@ var BoehmGC = Library{
 			// Use a minimal environment.
 			"-DNO_MSGBOX_ON_ERROR", // don't call MessageBoxA on Windows
 			"-DDONT_USE_ATEXIT",
+			"-DNO_CLOCK",           // don't use system clock
+			"-DNO_DEBUGGING",       // reduce code size
+			"-DGC_NO_FINALIZATION", // finalization is not used at the moment
+			"-DNO_GETENV",          // smaller binary, more predictable configuration
 
 			// Special flag to work around the lack of __data_start in ld.lld.
 			// TODO: try to fix this in LLVM/lld directly so we don't have to
@@ -38,6 +42,8 @@ var BoehmGC = Library{
 
 			// Do not scan the stack. We have our own mechanism to do this.
 			"-DSTACK_NOT_SCANNED",
+			"-DNO_PROC_STAT",  // we scan the stack manually (don't read /proc/self/stat on Linux)
+			"-DSTACKBOTTOM=0", // dummy value, we scan the stack manually
 
 			// Assertions can be enabled while debugging GC issues.
 			//"-DGC_ASSERTIONS",