build: fix sigsetjmp issues and ensure required libs on Linux

1. Handle `sigsetjmp` platform differences:
   - Separate `sigsetjmp` linkage to platform-specific files.
   - Use `__sigsetjmp` on Linux to handle `sigsetjmp` being a macro.
   - Maintain original implementation for Darwin.

2. Ensure linking of required libs:
   - Explicitly link against fundamental libs (e.g., libm, libatomic).
   - Address the fact that typical Linux linkers don't link these by
     default.

c/setjmp/setjmp_linux.go

@@ -0,0 +1,12 @@
+//go:build linux
+
+package setjmp
+
+import (
+	_ "unsafe"
+
+	"github.com/goplus/llgo/c"
+)
+
+//go:linkname Sigsetjmp C.__sigsetjmp
+func Sigsetjmp(env *SigjmpBuf, savemask c.Int) c.Int