fix(libkernel): map a protected main stack up to its top on Linux

A protect below the main thread's stack mapping mapped only the
protected pages. Linux keeps a growing stack one guard gap (1 MiB by
default) above a readable mapping, so after a read-only protect, or a
protect later restored to read/write, at the bottom of the reported
stack the stack could no longer grow into the megabyte above it and a
deep enough call faulted where the console has stack pages. The pages
between the protect and the top of the stack are now mapped as well,
so the stack never has to grow past them.

core/libs/tests/GuestKernelStackProtect.cpp

@@ -320,6 +320,7 @@ static int ProtectMainStackBottom(std::initializer_list<int> protections, std::u
         void* endAfter = nullptr;
         Require(sceKernelIsStack(&local, &startAfter, &endAfter) == SCE_OK && startAfter == start && endAfter == end);
     }
+    for (const auto offset : {GuardBytes, GuardBytes + 0x40000}) reinterpret_cast<volatile unsigned char*>(low + offset)[0] = 8;
     Require(QueryPage(low).raw == raw);
     return 0;
 }
@@ -347,7 +348,7 @@ static bool ChildPasses(const char* mode, bool unlimitedStack) {
 }
 
 static void ProtectMainStacks() {
-    Require(ChildPasses("--reprotect-bottom", false));
+    for (const char* mode : {"--read-only-bottom", "--reprotect-bottom"}) Require(ChildPasses(mode, false));
     rlimit limit{};
     Require(getrlimit(RLIMIT_STACK, &limit) == 0);
     if (limit.rlim_max != RLIM_INFINITY || OvercommitStrict()) {
@@ -365,6 +366,7 @@ int main(int argc, char** argv) {
     ProtectHostStacks();
 #else
     if (argc > 1 && std::strcmp(argv[1], "--unlimited-stack") == 0) return ProtectUnlimitedStack();
+    if (argc > 1 && std::strcmp(argv[1], "--read-only-bottom") == 0) return ProtectMainStackBottom({1}, 3);
     if (argc > 1 && std::strcmp(argv[1], "--reprotect-bottom") == 0) return ProtectMainStackBottom({0, 3}, 7);
     ProtectMainStacks();
     ProtectStack(0);