rust: compile in bootstrapping mode

Since we are going to use stable releases [*], we need to enable
the bootstrapping mode to use the unstable features.

[*] For the moment, it is a beta due to a fix we need for LLVM 12,
but the same logic applies.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Makefile

@@ -460,6 +460,7 @@ READELF		= $(CROSS_COMPILE)readelf
 STRIP		= $(CROSS_COMPILE)strip
 endif
 RUSTC		= rustc
+RUSTC_BOOTSTRAP = 1
 RUSTFMT		= rustfmt
 CLIPPY_DRIVER	= clippy-driver
 BINDGEN		= bindgen
@@ -548,7 +549,8 @@ ifdef RUST_LIB_SRC
 	export RUST_LIB_SRC
 endif
 
-export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE LD CC RUSTC BINDGEN
+export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE LD CC
+export RUSTC RUSTC_BOOTSTRAP BINDGEN
 export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AWK INSTALLKERNEL
 export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
 export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD