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.

internal/build/build.go

@@ -356,6 +356,8 @@ func linkMainPkg(ctx *context, pkg *packages.Package, pkgs []*aPackage, llFiles
 			"-rpath", "$ORIGIN",
 			"-rpath", "$ORIGIN/../lib",
 			"-Xlinker", "--gc-sections",
+			"-lm",
+			"-latomic",
 			"-lpthread", // libpthread is built-in since glibc 2.34 (2021-08-01); we need to support earlier versions.
 		)
 	}