share/mk: Substitute reproducible prefixes in dwarf info

Ensure that worlds/releases built from different source paths are
reproducible.  Currently they are not since src and obj paths are
embedded in DWARF info.  There are some uses of __FILE__ in the src tree
as well, and they are similarly handled by this change.

Use /usr/src and /usr/obj as the canonical src and obj paths when
WITH_REPRODUCIBLE_BUILD is defined.  Define a DEBUG_PREFIX variable
which is used to ask the compiler to substitute paths in output files
using -ffile-prefix-map.

Compat builds (i.e., lib32) complicate things since they override
OBJROOT to point to obj-${compat} but still refer to headers from the
native objroot (e.g., llvm headers).

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D50951

share/mk/bsd.debug.mk

@@ -13,6 +13,28 @@ CFLAGS+= -DNDEBUG
 MK_WERROR=	no
 .endif
 
+# If reproducible build mode is enabled, map the root of the source
+# directory to /usr/src and the root of the object directory to
+# /usr/obj.
+.if ${MK_REPRODUCIBLE_BUILD} != "no" && !defined(DEBUG_PREFIX)
+.if defined(SRCTOP)
+DEBUG_PREFIX+= ${SRCTOP:S,/$,,}=/usr/src
+.endif
+.if defined(OBJROOT)
+# Strip off compat subdirectories, e.g., /usr/obj/usr/src/amd64.amd64/obj-lib32
+# becomes /usr/obj/usr/src/amd64.amd64, since object files compiled there might
+# refer to something outside the root.
+DEBUG_PREFIX+= ${OBJROOT:S,/$,,:C,/obj-[^/]*$,,}=/usr/obj
+.endif
+.endif
+
+.if defined(DEBUG_PREFIX)
+.for map in ${DEBUG_PREFIX}
+CFLAGS+= -ffile-prefix-map=${map}
+CXXFLAGS+= -ffile-prefix-map=${map}
+.endfor
+.endif
+
 .if defined(DEBUG_FLAGS)
 CFLAGS+=${DEBUG_FLAGS}
 CXXFLAGS+=${DEBUG_FLAGS}

share/mk/bsd.opts.mk

@@ -78,6 +78,7 @@ __DEFAULT_NO_OPTIONS = \
     CCACHE_BUILD \
     CTF \
     INSTALL_AS_USER \
+    REPRODUCIBLE_BUILD \
     RETPOLINE \
     RUN_TESTS \
     STALE_STAGED \