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

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

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share/mk: Deduplicate some handling of debug info

bsd.prog.mk and bsd.lib.mk contain a bunch of duplicated logic used to
handle DEBUG_FLAGS and standalone debug file info (enabled by
MK_DEBUG_FILES)

share/mk: Deduplicate some handling of debug info

bsd.prog.mk and bsd.lib.mk contain a bunch of duplicated logic used to
handle DEBUG_FLAGS and standalone debug file info (enabled by
MK_DEBUG_FILES). In anticipation of adding more duplicated logic, let's
try factoring it out into a separate bsd.debug.mk first.

bsd.debug.mk now handles the following:
- MK_ASSERT_DEBUG
- installation rules for debug files (the consumer has to set DEBUGFILE)
- updating CFLAGS and CXXFLAGS based on DEBUG_FLAGS
- optionally stripping installed files

No functional change intended.

Reviewed by: bdrewery
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D51805

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