Cygwin: link mcountFunc.o to libgmon.a

Commit 188d5f6c9a 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: 188d5f6c9a ("Cygwin: x86_64: add wmemset assembler entry point")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/Makefile.am

@@ -55,8 +55,7 @@ TEST_LIB_NAME=libcygwin0.a
 if TARGET_X86_64
 TARGET_FILES= \
 	x86_64/memcpy.s \
-	x86_64/memset.s \
-	x86_64/mcountFunc.S
+	x86_64/memset.s
 endif
 
 LIB_FILES= \
@@ -354,7 +353,13 @@ DLL_FILES= \
 	window.cc \
 	winf.cc
 
+if TARGET_X86_64
+GMON_TARGET_FILES = \
+	x86_64/mcountFunc.S
+endif
+
 GMON_FILES= \
+	$(GMON_TARGET_FILES) \
 	gmon.c \
 	mcount.c \
 	profil.c

winsup/cygwin/release/3.4.1

@@ -0,0 +1,6 @@
+Bug Fixes
+---------
+
+- Fix an error introduced into the build process, resulting in `gcc -pg'
+  becoming disfunctional.
+  Addresses: https://cygwin.com/pipermail/cygwin/2022-December/252619.html