Enable and make stage0 landing pads optional

configure

@@ -578,6 +578,7 @@ opt ccache 0 "invoke gcc/clang via ccache to reuse object files between builds"
 opt local-rust 0 "use an installed rustc rather than downloading a snapshot"
 opt llvm-static-stdcpp 0 "statically link to libstdc++ for LLVM"
 opt rpath 0 "build rpaths into rustc itself"
+opt stage0-landing-pads 1 "enable landing pads during bootstrap with stage0"
 # This is used by the automation to produce single-target nightlies
 opt dist-host-only 0 "only install bins for the host architecture"
 opt inject-std-version 1 "inject the current compiler version of libstd into programs"

mk/main.mk

@@ -170,7 +170,10 @@ RUST_LIB_FLAGS_ST3 += -C prefer-dynamic
 
 # Landing pads require a lot of codegen. We can get through bootstrapping faster
 # by not emitting them.
-RUSTFLAGS_STAGE0 += -Z no-landing-pads
+
+ifdef CFG_DISABLE_STAGE0_LANDING_PADS
+  RUSTFLAGS_STAGE0 += -Z no-landing-pads
+endif
 
 # Enable MIR to "always build" for crates where this works. This is
 # just temporary while MIR is being actively built up -- it's just a