compiler: use std.heap.smp_allocator

In main, now this allocator is chosen by default when compiling without
libc in ReleaseFast or ReleaseSmall, and not targeting WebAssembly.

bootstrap.c

@@ -139,7 +139,7 @@ int main(int argc, char **argv) {
             "pub const enable_tracy = false;\n"
             "pub const value_tracing = false;\n"
             "pub const skip_non_native = false;\n"
-            "pub const force_gpa = false;\n"
+            "pub const debug_gpa = false;\n"
             "pub const dev = .core;\n"
             "pub const value_interpret_mode = .direct;\n"
         , zig_version);