build: use full path when compiling libraries

Makes gdb display full paths in backtrace.
Dropped during the most recent reorg.

R=r
CC=golang-dev
https://golang.org/cl/2216047

src/Make.clib

@@ -7,7 +7,7 @@
 all: $(LIB)
 
 %.$(HOST_O): %.c
-	$(HOST_CC) $(HOST_CFLAGS) -c $*.c
+	$(HOST_CC) $(HOST_CFLAGS) -c "$(PWD)/$*.c"
 
 $(OFILES): $(HFILES)