add shared library support

This adds support for building WASI shared libraries per
https://github.com/WebAssembly/tool-conventions/blob/main/DynamicLinking.md.

For the time being, the goal is to allow "pseudo-dynamic" linking using the
Component Model per
https://github.com/WebAssembly/component-model/blob/main/design/mvp/examples/SharedEverythingDynamicLinking.md.
This requires all libraries to be available when the component is created, but
still allows runtime symbol resolution via `dlopen`/`dlsym` backed by a static
lookup table.  This is sufficient to support Python native extensions, for
example.  A complete demo using `wit-component` is available at
https://github.com/dicej/component-linking-demo.

This commit adds support for building `libc.so`, `libc++.so`, and `libc++abi.so`
alongside their static counterparts.

Notes:

- I had to refactor `errno` support a bit to avoid a spurious `_ZTH5errno` (AKA "thread-local initialization routine for errno") import in `libc++.so`.
- Long double print and scan are included by default in `libc.so` rather than in a separate library.
- `__main_argc_argv` is now a weak symbol since it's not relevant for reactors.
- `dlclose`/`dlsym` rely on a lookup table provided by the "dynamic" linker via `__wasm_set_libraries`.  Not all flags are supported yet, and unrecognized flags will result in an error.
- This requires https://reviews.llvm.org/D153293, which we will need to backport to LLVM 16 until 17 is released.  I'll open a `wasi-sdk` PR with that change and various Makefile tweaks to support shared libraries.
- `libc.so` is temporarily disabled for the `wasi-threads` build until someone can make `wasi_thread_start.s` position-independent.

Signed-off-by: Joel Dice <[email protected]>

expected/wasm32-wasi-threads/defined-symbols.txt

@@ -49,6 +49,7 @@ __do_orphaned_stdio_locks
 __dummy_reference
 __duplocale
 __env_rm_add
+__errno
 __errno_location
 __exp2f_data
 __exp_data
@@ -401,6 +402,7 @@ __wasilibc_tell
 __wasilibc_unlinkat
 __wasilibc_utimens
 __wasm_call_dtors
+__wasm_set_libraries
 __wcscoll_l
 __wcsftime_l
 __wcsxfrm_l
@@ -573,6 +575,10 @@ difftime
 dirfd
 dirname
 div
+dlclose
+dlerror
+dlopen
+dlsym
 dprintf
 drand48
 drem
@@ -588,7 +594,6 @@ erfcf
 erfcl
 erff
 erfl
-errno
 exit
 exp
 exp10

expected/wasm32-wasi-threads/include-all.c

@@ -75,6 +75,7 @@
 #include <crypt.h>
 #include <ctype.h>
 #include <dirent.h>
+#include <dlfcn.h>
 #include <endian.h>
 #include <err.h>
 #include <errno.h>