In #1622, when targeting WebAsembly, the --allow-undefined flag
became unconditionally added to the linker.

This is not always desirable.

First, this is error prone. Code with references to unkown symbols
will link just fine, but then fail at run-time.

This behavior is inconsistent with all other targets.

For freestanding wasm applications, and applications that only use
WASI, undefined references are better reported at compile-time.

This behavior is also inconsistent with clang itself. Autoconf and
cmake scripts checking for function presence think that all tested
functions exist, but then resulting application cannot run.

For example, this is one of the reasons compilation of Ruby 3.2.0
to WASI fails with zig cc, while it works out of the box with clang.
But all applications checking for symbol existence before compilation
are affected.

This reverts the behavior to the one Zig had before #1622, and
introduces an `import_symbols` flag to ignore undefined symbols,
assuming that the webassembly runtime will define them.