Cygwin: link mcountFunc.o to libgmon.a

Cygwin: link mcountFunc.o to libgmon.a

Commit 188d5f6c9ad5 erroneously moved the mcountFunc.S file to the
TARGET_FILES target, rather than keeping it in GMON_FILES.  The
result is that the __fentry__ entry point is now entirely undefined,
so `gcc -pg' is broken.

Create new target-specific GMON_TARGET_FILES and move mcountFunc.S
into it.  Add $(GMON_TARGET_FILES) to GMON_FILES.

Fixes: 188d5f6c9ad5 ("Cygwin: x86_64: add wmemset assembler entry point")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>