Fix build problems of btrfs and packagefs for x86_64.

* Both filesystems used to link to a static kernel-zlib, which 
  was being built with -fno-pic. This doesn't work on x86_64 as the 
  filesystem add-ons are meant to be relocatable, which requires their 
  code to be compiled as position independent.
  Solve that by moving zlib into the kernel, so any add-on can just use
  it from there (packagefs is mandatory, so we can't really do without 
  zlib anyway).

src/add-ons/kernel/file_systems/btrfs/Jamfile

@@ -18,5 +18,4 @@ KernelAddon btrfs :
 	DirectoryIterator.cpp
 	Inode.cpp
 	Volume.cpp
-	: kernel_libz.a
 ;

src/add-ons/kernel/file_systems/packagefs/Jamfile

@@ -6,7 +6,7 @@ UsePrivateKernelHeaders ;
 UsePrivateHeaders package shared storage ;
 
 
-local subDirs = 
+local subDirs =
 	indices
 	nodes
 	package
@@ -123,7 +123,7 @@ KernelAddon packagefs
 	$(HAIKU_PACKAGE_FS_PACKAGE_READER_SOURCES_V1)
 	$(libSharedSources)
 
-	: $(TARGET_KERNEL_LIBSUPC++) kernel_libz.a
+	: $(TARGET_KERNEL_LIBSUPC++)
 ;
 
 

src/system/kernel/Jamfile

@@ -128,6 +128,7 @@ KernelLd kernel_$(TARGET_ARCH) :
 	kernel_lib_posix.o
 	kernel_lib_posix_arch_$(TARGET_ARCH).o
 	kernel_misc.o
+	kernel_zlib.o
 
 	$(TARGET_KERNEL_LIBSUPC++)
 
@@ -170,6 +171,7 @@ if $(HAIKU_ARCH) = x86_64 {
 		kernel_lib_posix.o
 		kernel_lib_posix_arch_$(TARGET_ARCH).o
 		kernel_misc.o
+		kernel_zlib.o
 
 		$(TARGET_KERNEL_LIBSUPC++)
 

src/system/kernel/lib/zlib/Jamfile

@@ -17,7 +17,7 @@ LOCATE on [ FGristFiles $(zlibSources) ] = $(zlibSourceDirectory) ;
 Depends [ FGristFiles $(zlibSources) ]
 	: [ BuildFeatureAttribute zlib : sources ] ;
 
-KernelStaticLibrary kernel_libz.a :
+KernelMergeObject kernel_zlib.o :
 	$(zlibSources)
-	: -fno-pic
+	: $(TARGET_KERNEL_PIC_CCFLAGS)
 	;