[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/j6/include/j6/channel.hh

@@ -8,10 +8,11 @@
 
 #include <stddef.h>
 #include <j6/types.h>
+#include <util/api.h>
 
 namespace j6 {
 
-class channel
+class API channel
 {
 public:
     /// Create a new channel of the given size.

src/libraries/j6/include/j6/init.h

@@ -4,6 +4,7 @@
 
 #include <stdint.h>
 #include <j6/types.h>
+#include <util/api.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -71,7 +72,7 @@ struct j6_init_args
 
 
 /// Find the first handle of the given type held by this process
-j6_handle_t j6_find_first_handle(j6_object_type obj_type);
+j6_handle_t API j6_find_first_handle(j6_object_type obj_type);
 
 /// Get the init args
 const j6_init_args * j6_get_init_args();

src/libraries/j6/include/j6/memutils.h

@@ -2,6 +2,7 @@
 /// \file memutils.h
 /// Standard mem*() library functions
 
+#include <util/api.h>
 #include <__j6libc/restrict.h>
 #include <__j6libc/size_t.h>
 
@@ -9,9 +10,9 @@
 extern "C" {
 #endif
 
-void *memcpy(void * restrict s1, const void * restrict s2, size_t n);
-void *memmove(void * restrict s1, const void * restrict s2, size_t n);
-void *memset(void *s, int c, size_t n);
+void * API memcpy(void * restrict s1, const void * restrict s2, size_t n);
+void * API memmove(void * restrict s1, const void * restrict s2, size_t n);
+void * API memset(void *s, int c, size_t n);
 
 #ifdef __cplusplus
 } // extern "C"

src/libraries/j6/include/j6/protocols/service_locator.hh

@@ -1,9 +1,10 @@
 #include <j6/protocols/service_locator.h>
 #include <j6/types.h>
+#include <util/api.h>
 
 namespace j6::proto::sl {
 
-class client
+class API client
 {
 public:
     /// Constructor.

src/libraries/j6/include/j6/protocols/vfs.hh

@@ -1,9 +1,14 @@
+#pragma once
+/// \file vfs.hh
+/// C++ client interface for VFS protocol
+
 #include <j6/protocols/vfs.h>
 #include <j6/types.h>
+#include <util/api.h>
 
 namespace j6::proto::vfs {
 
-class client
+class API client
 {
 public:
     /// Constructor.

src/libraries/j6/include/j6/sysconf.h.cog

@@ -10,6 +10,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include <util/api.h>
 
 /*[[[cog code generation
 from os.path import join
@@ -35,7 +36,7 @@ enum j6_sysconf_arg
 
 /// Get the kernel configuration value specified by
 /// the argument.
-unsigned long j6_sysconf(j6_sysconf_arg arg);
+unsigned long API j6_sysconf(j6_sysconf_arg arg);
 
 #ifdef __cplusplus
 } // extern C

src/libraries/j6/include/j6/syslog.hh

@@ -2,13 +2,15 @@
 /// \file j6/syslog.hh
 /// Utility function for writing messages to the kernel log
 
+#include <util/api.h>
+
 // The kernel depends on libj6 for some shared code,
 // but should not include the user-specific code.
 #ifndef __j6kernel
 
 namespace j6 {
 
-void syslog(const char *fmt, ...);
+void API syslog(const char *fmt, ...);
 
 } // namespace j6
 

src/libraries/j6/init.cpp

@@ -36,13 +36,13 @@ j6_find_first_handle(j6_object_type obj_type)
     return j6_handle_invalid;
 }
 
-const j6_init_args *
+const j6_init_args * API
 j6_get_init_args()
 {
     return &init_args;
 }
 
-extern "C" void
+extern "C" void API
 __init_libj6(uint64_t arg0, uint64_t arg1)
 {
     init_args.args[0] = arg0;

src/libraries/j6/syslog.cpp

@@ -4,6 +4,7 @@
 
 #include <util/format.h>
 #include <j6/syscalls.h>
+#include <j6/syslog.hh>
 
 namespace j6 {