lang/rust-bootstrap: Add option that doesn't depend on freebsd-sysroot

Hello.

rust-1.79.0 is the last version we can build with 12.4-STABLE (and also 13.2?).
Because the bootstrap compiler cannot be used due to the following output.

===>  Building for rust-1.80.1
Building bootstrap
running: /wrkdirs/usr/ports/lang/rust/work/bootstrap/bin/cargo build --manifest-path /wrkdirs/usr/ports/lang/rust/work/rustc-1.80.1-src/src/bootstrap/Cargo.toml --verbose --verbose --frozen
error: process didn't exit successfully: `/wrkdirs/usr/ports/lang/rust/work/bootstrap/bin/rustc -vV` (exit status: 1)
--- stderr
ld-elf.so.1: /wrkdirs/usr/ports/lang/rust/work/bootstrap/bin/../lib/librustc_driver-46295f53743f17e0.so: Undefined symbol "_ZNSt3__122__libcpp_verbose_abortEPKcz"

However, I succeeded in building rust-1.80.1 on 12.4-STABLE in roughly the following way.

mkdir -p -- /usr/local/freebsd-sysroot/amd64/bin
ln -s /usr/bin/cc /usr/local/freebsd-sysroot/amd64/bin/cc
ln -s /usr/bin/c++ /usr/local/freebsd-sysroot/amd64/bin/c++
Edit lang/rust-bootstrap/Makefile so that it does not depend on devel/freebsd-sysroot.
make -C /usr/ports/lang/rust-bootstrap/ install FLAVOR=amd64
cp -f /usr/local/rust-bootstrap/amd64/* ${DISTDIR}/rust/2024-06-13/
Edit sha256 and size of lang/rust/distinfo to regenerated ones.
Start building rust-1.80.1.

This is not a request for continued support for versions that are no longer supported.
This means that I need a way to get a bootstrap compiler that is made in the same environment as the one we are currently using.
It may make it easier to find a solution to a problem that is rare these days, where rust doesn't work depending on the architecture.

So, I'm going to paste the patch I made below :)
I also feel like I'm doing something wrong about it, so please make it without using it directly.

Regards.
Here's the patch:

diff --git a/lang/rust-bootstrap/Makefile b/lang/rust-bootstrap/Makefile
index 3846aa32ffe..88c1c5028e3 100644
--- a/lang/rust-bootstrap/Makefile
+++ b/lang/rust-bootstrap/Makefile
@@ -22,8 +22,7 @@ LICENSE_FILE_MIT=	${WRKSRC}/LICENSE-MIT
 ONLY_FOR_ARCHS=	amd64 powerpc64 powerpc64le
 ONLY_FOR_ARCHS_REASON=	untested on other architectures

-BUILD_DEPENDS=	${FLAVOR:S/_/-/g}-freebsd-sysroot>=a2021.09.14:devel/freebsd-sysroot@${FLAVOR} \
-		cmake:devel/cmake-core \
+BUILD_DEPENDS=	cmake:devel/cmake-core \
 		gmake:devel/gmake \
 		rust>=${PORTVERSION}:lang/rust

@@ -43,6 +42,12 @@ TMPDIR?=	${WRKDIR}
 # Resulting packages are not specific to amd64
 NO_ARCH=	yes

+OPTIONS_DEFINE=	SYSROOT
+OPTIONS_DEFAULT=	SYSROOT
+SYSROOT_DESC=	Depend on freebsd-sysroot
+SYSROOT_BUILD_DEPENDS=	${FLAVOR:S/_/-/g}-freebsd-sysroot>=a2021.09.14:devel/freebsd-sysroot@${FLAVOR}
+SYSROOT_VARS_OFF=	ONLY_FOR_ARCHS+="aarch64"
+
 _CARGO_VENDOR_DIR=			${WRKSRC}/vendor
 _RUST_ARCH_amd64=			x86_64
 _RUST_ARCH_i386=			i686
@@ -70,6 +75,10 @@ IGNORE=	is only for FreeBSD
 EXTRA_PATCHES+=	${PATCHDIR}/${FLAVOR:S/_/-/}
 .endif

+.if ! ${PORT_OPTIONS:MSYSROOT} && ${_RUST_HOST} != ${_RUST_TARGET}
+BROKEN=	The behavior of this condition is not implemented.
+.endif
+
 post-patch:
 # Disable vendor checksums
 	@${REINPLACE_CMD} 's,"files":{[^}]*},"files":{},' \
@@ -110,9 +119,15 @@ do-configure:
 # host compiler that can build the host->target compiler
 	@${ECHO_CMD} 'targets="${_RUST_LLVM_TARGET};${_RUST_LLVM_TARGET_${ARCH}}"' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} '[target.${_RUST_TARGET}]' >> ${WRKSRC}/config.toml
+.if ${PORT_OPTIONS:MSYSROOT}
 	@${ECHO_CMD} 'cc="${LOCALBASE}/freebsd-sysroot/${FLAVOR:S/_/-/g}/bin/cc"' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'cxx="${LOCALBASE}/freebsd-sysroot/${FLAVOR:S/_/-/g}/bin/c++"' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'linker="${LOCALBASE}/freebsd-sysroot/${FLAVOR:S/_/-/g}/bin/cc"' >> ${WRKSRC}/config.toml
+.else
+	@${ECHO_CMD} 'cc="${CC}"' >> ${WRKSRC}/config.toml
+	@${ECHO_CMD} 'cxx="${CXX}"' >> ${WRKSRC}/config.toml
+	@${ECHO_CMD} 'linker="${CC}"' >> ${WRKSRC}/config.toml
+.endif
 .for _key _util in ar ${AR} ranlib ${RANLIB}
 	@bin="$$(which ${_util})"; \
 		${ECHO_CMD} "${_key}=\"$$bin\"" >> ${WRKSRC}/config.toml