improvements to build-lib use case of WebAssembly

 * build-exe does include the startup code that supplies _start for the
   wasm32-freestanding target. Previously this did not occur because
   of logic excluding "freestanding".
 * build-lib for wasm32-freestanding target gets linked by LLD. To avoid
   infinite recursion, compiler_rt and zig libc are built as objects
   rather than libraries.
   - no "lib" prefix and ".wasm" extension instead of ".a". Rather than
   build-lib foo.zig producing "libfoo.a", now it produces "foo.wasm".
 * go back to using `.o` extension for webassembly objects
 * zig libc only provides _start symbol for wasm when linking libc.

2 files changed, 8 insertions, 4 deletions

diff --git a/std/special/bootstrap.zig b/std/special/bootstrap.zig
index e6505c836b..e8f943b78c 100644
--- a/std/special/bootstrap.zig
+++ b/std/special/bootstrap.zig
@@ -25,21 +25,25 @@ nakedcc fn _start() noreturn {
     }
 
     switch (builtin.arch) {
-        builtin.Arch.x86_64 => {
+        .x86_64 => {
             argc_ptr = asm ("lea (%%rsp), %[argc]"
                 : [argc] "=r" (-> [*]usize)
             );
         },
-        builtin.Arch.i386 => {
+        .i386 => {
             argc_ptr = asm ("lea (%%esp), %[argc]"
                 : [argc] "=r" (-> [*]usize)
             );
         },
-        builtin.Arch.aarch64, builtin.Arch.aarch64_be => {
+        .aarch64, .aarch64_be => {
             argc_ptr = asm ("mov %[argc], sp"
                 : [argc] "=r" (-> [*]usize)
             );
         },
+        .wasm32, .wasm64 => {
+            _ = callMain();
+            while (true) {}
+        },
         else => @compileError("unsupported arch"),
     }
     // If LLVM inlines stack variables into _start, they will overwrite
diff --git a/std/special/c.zig b/std/special/c.zig
index d5de52a6a3..281e177469 100644
--- a/std/special/c.zig
+++ b/std/special/c.zig
@@ -11,7 +11,7 @@ const maxInt = std.math.maxInt;
 const is_wasm = switch (builtin.arch) { .wasm32, .wasm64 => true, else => false};
 const is_freestanding = switch (builtin.os) { .freestanding => true, else => false };
 comptime {
-    if (is_freestanding and is_wasm) {
+    if (is_freestanding and is_wasm and builtin.link_libc) {
         @export("_start", wasm_start, .Strong);
     }
 }