lang/rust: Fix DOCS build, unbreak RUST_BACKTRACE

- Fix DOCS build by disabling deny-warnings (rustc equivalent to
  -Werror).  Let's not fail the entire build just because a handful
  of docs fail to build due to -D broken-intra-doc-links [0]
- Only install the host target docs.  We otherwise waste time with
  a rust-docs install-uninstall-install cycle for each additional
  target during stage.
- Backport FreeBSD support for gimli, unbreak RUST_BACKTRACE, and
  have useful stack traces with actual content again [1]
- While here also strip binaries under libexec/ and ack reading of
  src/bootstrap/CHANGELOG.md (changelog-seen=2)

[0] rust-lang/rust#76526

PR:		253557 [1]
Submitted by:	jbeich [1]

lang/rust-bootstrap/Makefile

@@ -2,6 +2,7 @@
 
 PORTNAME=	rust
 PORTVERSION=	1.50.0
+PORTREVISION=	1
 CATEGORIES=	lang
 MASTER_SITES=	https://static.rust-lang.org/dist/:rust \
 		LOCAL/mikael:armbase \

lang/rust/Makefile

@@ -3,7 +3,7 @@
 
 PORTNAME=	rust
 PORTVERSION?=	1.50.0
-PORTREVISION?=	0
+PORTREVISION?=	1
 CATEGORIES=	lang
 MASTER_SITES=	https://static.rust-lang.org/dist/:src \
 		https://dev-static.rust-lang.org/dist/:src \
@@ -46,7 +46,6 @@ CONFLICTS_INSTALL?=	rust-nightly
 
 OPTIONS_DEFINE=		DOCS GDB SOURCES WASM
 OPTIONS_DEFAULT=	SOURCES WASM
-OPTIONS_EXCLUDE=	DOCS # https://github.com/rust-lang/rust/issues/76526
 .if defined(NIGHTLY_DATE)
 OPTIONS_EXCLUDE+=	SOURCES
 .endif
@@ -122,7 +121,8 @@ do-configure:
 	@cd ${WRKDIR}/${_component}-*-${OPSYS:tl} && \
 		${SH} install.sh --prefix=${WRKDIR}/bootstrap --verbose
 .endfor
-	@${ECHO_CMD} '[build]' > ${WRKSRC}/config.toml
+	@${ECHO_CMD} 'changelog-seen=2' > ${WRKSRC}/config.toml
+	@${ECHO_CMD} '[build]' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'vendor=true' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'extended=true' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'python="${PYTHON_CMD}"' >> ${WRKSRC}/config.toml
@@ -141,6 +141,7 @@ do-configure:
 	@${ECHO_CMD} '[rust]' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'channel="${PKGNAMESUFFIX:Ustable:S/^-//}"' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'default-linker="${CC}"' >> ${WRKSRC}/config.toml
+	@${ECHO_CMD} 'deny-warnings=false' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'verbose-tests=true' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'lld=${_RUST_BUILD_WASM}' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} '[llvm]' >> ${WRKSRC}/config.toml
@@ -196,12 +197,19 @@ do-install:
 		${STAGEDIR}${PREFIX}/lib/rustlib/install.log \
 		${STAGEDIR}${PREFIX}/lib/rustlib/manifest-* \
 		${STAGEDIR}${PREFIX}/lib/rustlib/uninstall.sh
-	@${FIND} ${STAGEDIR}${PREFIX}/bin ${STAGEDIR}${PREFIX}/lib -exec ${FILE} -i {} + | \
+	@${FIND} ${STAGEDIR}${PREFIX}/bin ${STAGEDIR}${PREFIX}/lib \
+		${STAGEDIR}${PREFIX}/libexec -exec ${FILE} -i {} + | \
 		${AWK} -F: '/executable|sharedlib/ { print $$1 }' | ${XARGS} ${STRIP_CMD}
 	@${FIND} ${STAGEDIR}${PREFIX} -not -type d | \
 		${SED} -E -e 's,^${STAGEDIR}${PREFIX}/,,' \
 			-e 's,(share/man/man[1-9]/.*\.[0-9]),\1.gz,' >> ${TMPPLIST}
 
+post-install-DOCS-on:
+# Ignore any left behind empty directories in case some docs fail
+# to build (failures are ignored due to deny-warnings=false).
+	@${FIND} ${STAGEDIR}${DOCSDIR}/html -empty -type d | \
+		${SED} 's,^${STAGEDIR},@comment @dir ,' >> ${TMPPLIST}
+
 post-install-SOURCES-on:
 # Silence stage-qa warnings by sanitizing permissions on sources
 	@${FIND} ${STAGEDIR}${PREFIX}/lib/rustlib/src -type f -exec ${CHMOD} \