add realpath.c to `LIBC_TOP_HALF_MUSL_SOURCES` (#473)

* provide a `realpath` stub

In https://github.com/WebAssembly/wasi-libc/pull/463, I added stubs for
`statvfs`, `chmod`, etc. but forgot to add one for `realpath`, which is also
required by `libc++`'s `<filesystem>` implementation.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* remove `realpath` stub and use musl's version instead

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

---------

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Makefile

@@ -127,6 +127,7 @@ LIBC_TOP_HALF_MUSL_SOURCES = \
         misc/getopt.c \
         misc/getopt_long.c \
         misc/getsubopt.c \
+        misc/realpath.c \
         misc/uname.c \
         misc/nftw.c \
         errno/strerror.c \

expected/wasm32-wasi-preview2/defined-symbols.txt

@@ -1050,6 +1050,7 @@ readlinkat
 readv
 realloc
 reallocarray
+realpath
 recv
 regcomp
 regerror

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

@@ -1084,6 +1084,7 @@ readlinkat
 readv
 realloc
 reallocarray
+realpath
 recv
 regcomp
 regerror

expected/wasm32-wasi/defined-symbols.txt

@@ -941,6 +941,7 @@ readlinkat
 readv
 realloc
 reallocarray
+realpath
 recv
 regcomp
 regerror