Preserve file timestamps when copying files in the install target (#581)

This change makes it so that the timestamps of installed header files
are only updated when the contents of the header files change. This
change is beneficial for incremental builds of projects depending on
wasi-libc. For example, the Swift project builds wasi-libc as part of
its build process, and this change reduces the number of build products
in the Swift build that are unnecessarily rebuilt.

This is a 2nd attempt of
https://github.com/WebAssembly/wasi-libc/pull/530 but much simpler
thanks to @abrown 's build work

Makefile

@@ -816,7 +816,7 @@ endif
 
 install: finish
 	mkdir -p "$(INSTALL_DIR)"
-	cp -r "$(SYSROOT)/lib" "$(SYSROOT)/share" "$(SYSROOT)/include" "$(INSTALL_DIR)"
+	cp -p -r "$(SYSROOT)/lib" "$(SYSROOT)/share" "$(SYSROOT)/include" "$(INSTALL_DIR)"
 
 DEFINED_SYMBOLS = $(SYSROOT_SHARE)/defined-symbols.txt
 UNDEFINED_SYMBOLS = $(SYSROOT_SHARE)/undefined-symbols.txt