wasm: update wasi-libc dependency

The latest version allows overriding the default CFLAGS. By default,
they're `-O2 -DNDEBUG`, thus not including DWARF debug information. This
commit changes this to include the `-g` flag.

Apart from an improved debug experience, this lets -size=full attribute
code to wasi-libc.

Before:

    $ tinygo build -o test.wasm -size=full ./testdata/alias.go
       code  rodata    data     bss |   flash     ram | package
    ------------------------------- | --------------- | -------
       1780       0     188  130733 |    1968  130921 | (unknown)
         84       0       0       0 |      84       0 | internal/task
        281       0       0       0 |     281       0 | main
       2374       0       4     147 |    2378     151 | runtime
    ------------------------------- | --------------- | -------
       4519       0     192  130880 |    4711  131072 | total

After:

    $ tinygo build -o test.wasm -size=full ./testdata/alias.go
       code  rodata    data     bss |   flash     ram | package
    ------------------------------- | --------------- | -------
         40       0     188  130733 |     228  130921 | (unknown)
       1740       0       0       0 |    1740       0 | C wasi-libc
         84       0       0       0 |      84       0 | internal/task
        281       0       0       0 |     281       0 | main
       2374       0       4     147 |    2378     151 | runtime
    ------------------------------- | --------------- | -------
       4519       0     192  130880 |    4711  131072 | total

The main difference here is the `(unknown)` code, which turns out to be
mostly wasi-libc in this trivial example.

Makefile

@@ -175,7 +175,7 @@ $(LLVM_BUILDDIR): $(LLVM_BUILDDIR)/build.ninja
 wasi-libc: lib/wasi-libc/sysroot/lib/wasm32-wasi/libc.a
 lib/wasi-libc/sysroot/lib/wasm32-wasi/libc.a:
 	@if [ ! -e lib/wasi-libc/Makefile ]; then echo "Submodules have not been downloaded. Please download them using:\n  git submodule update --init"; exit 1; fi
-	cd lib/wasi-libc && make -j4 WASM_CC=$(CLANG) WASM_AR=$(LLVM_AR) WASM_NM=$(LLVM_NM)
+	cd lib/wasi-libc && make -j4 WASM_CFLAGS="-O2 -g -DNDEBUG" WASM_CC=$(CLANG) WASM_AR=$(LLVM_AR) WASM_NM=$(LLVM_NM)
 
 
 # Build the Go compiler.

lib/wasi-libc

@@ -1 +1 @@
-Subproject commit 215adc8ac9f91eb055311acc72683fd2eb1ae15a
+Subproject commit ad5133410f66b93a2381db5b542aad5e0964db96