gc: drop support for 'precise' globals

Precise globals require a whole program optimization pass that is hard
to support when building packages separately. This patch removes support
for these globals by converting the last use (Linux) to use
linker-defined symbols instead.

For details, see: https://github.com/tinygo-org/tinygo/issues/2870

src/runtime/os_linux.go

@@ -1,8 +1,13 @@
-//go:build linux
-// +build linux
+//go:build linux && !baremetal && !nintendoswitch && !wasi
+// +build linux,!baremetal,!nintendoswitch,!wasi
 
 package runtime
 
+// This file is for systems that are _actually_ Linux (not systems that pretend
+// to be Linux, like baremetal systems).
+
+import "unsafe"
+
 const GOOS = "linux"
 
 const (
@@ -18,3 +23,20 @@ const (
 	clock_REALTIME      = 0
 	clock_MONOTONIC_RAW = 4
 )
+
+//go:extern _edata
+var globalsStartSymbol [0]byte
+
+//go:extern _end
+var globalsEndSymbol [0]byte
+
+// markGlobals marks all globals, which are reachable by definition.
+//
+// This implementation marks all globals conservatively and assumes it can use
+// linker-defined symbols for the start and end of the .data section.
+func markGlobals() {
+	start := uintptr(unsafe.Pointer(&globalsStartSymbol))
+	end := uintptr(unsafe.Pointer(&globalsEndSymbol))
+	start = (start + unsafe.Alignof(uintptr(0)) - 1) &^ (unsafe.Alignof(uintptr(0)) - 1) // align on word boundary
+	markRoots(start, end)
+}