[build] Address symbol visibility and DSO builds

Added an `API` macro in `j6/api.h` that expands to mark the given
declaration as a default-visible symbol. Also change `format` and
`vformat` to non-template functions, and make calls to `main`, `exit`,
and the library init functions in `_start` GOT-relative.

src/libraries/util/format.cpp

@@ -88,7 +88,6 @@ append_string(char_t *&out, size_t &count, size_t max, unsigned width, char_t co
     }
 }
 
-} // namespace
 
 template <typename char_t> size_t
 vformat(counted<char_t> output, char_t const *format, va_list va)
@@ -169,20 +168,28 @@ vformat(counted<char_t> output, char_t const *format, va_list va)
     return count;
 }
 
-template <typename char_t> size_t
-format(counted<char_t> output, const char_t *format, ...)
+} // namespace
+
+size_t API format(counted<char> output, const char *format, ...)
 {
     va_list va;
     va_start(va, format);
-    size_t result = vformat<char_t>(output, format, va);
+    size_t result = vformat<char>(output, format, va);
     va_end(va);
     return result;
 }
 
-template size_t format<char>(counted<char> output, const char *format, ...);
-template size_t vformat<char>(counted<char> output, const char *format, va_list va);
+size_t API format(counted<wchar_t> output, const wchar_t *format, ...)
+{
+    va_list va;
+    va_start(va, format);
+    size_t result = vformat<wchar_t>(output, format, va);
+    va_end(va);
+    return result;
+}
+
+size_t API vformat(counted<char> output, const char *format, va_list va) { return vformat<char>(output, format, va); }
+size_t API vformat(counted<wchar_t> output, const wchar_t *format, va_list va) { return vformat<wchar_t>(output, format, va); }
 
-template size_t format<wchar_t>(counted<wchar_t> output, const wchar_t *format, ...);
-template size_t vformat<wchar_t>(counted<wchar_t> output, const wchar_t *format, va_list va);
 
 } //namespace util

src/libraries/util/include/util/allocator.h

@@ -2,6 +2,9 @@
 /// \file allocator.h
 /// Definition of the allocator interface.
 
+#include <stddef.h>
+#include <j6/memutils.h>
+
 namespace util {
 
 // Allocators are types with three static methods with these signatures

src/libraries/util/include/util/api.h

@@ -0,0 +1,7 @@
+#pragma once
+/// \file api.h
+/// Library interface visibility macros
+
+#ifndef API
+#define API __attribute__ ((visibility ("default")))
+#endif

src/libraries/util/include/util/bip_buffer.h

@@ -6,11 +6,12 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <util/api.h>
 #include <util/spinlock.h>
 
 namespace util {
 
-class bip_buffer
+class API bip_buffer
 {
 public:
     /// Default constructor. Creates a zero-size buffer.

src/libraries/util/include/util/format.h

@@ -4,14 +4,16 @@
 #pragma once
 
 #include <stdarg.h>
+#include <util/api.h>
 #include <util/counted.h>
 
 namespace util {
 
-template <typename char_t>
-size_t format(counted<char_t> output, const char_t *format, ...);
+size_t API format(counted<char> output, const char *format, ...);
+size_t API vformat(counted<char> output, const char *format, va_list va);
 
-template <typename char_t>
-size_t vformat(counted<char_t> output, const char_t *format, va_list va);
+
+size_t API format(counted<wchar_t> output, const wchar_t *format, ...);
+size_t API vformat(counted<wchar_t> output, const wchar_t *format, va_list va);
 
 }

src/libraries/util/include/util/spinlock.h

@@ -3,6 +3,7 @@
 #pragma once
 
 #include <stdint.h>
+#include <util/api.h>
 
 #ifdef __j6kernel
 extern "C" uint32_t __current_thread_id();
@@ -11,7 +12,7 @@ extern "C" uint32_t __current_thread_id();
 namespace util {
 
 /// An MCS based spinlock
-class spinlock
+class API spinlock
 {
 public:
     spinlock();

src/libraries/util/util.module

@@ -10,6 +10,7 @@ module("util",
     ],
     public_headers = [
         "util/allocator.h",
+        "util/api.h",
         "util/assert.h",
         "util/basic_types.h",
         "util/bip_buffer.h",