crate-ify compiler-rt into compiler-builtins

libcompiler-rt.a is dead, long live libcompiler-builtins.rlib

This commit moves the logic that used to build libcompiler-rt.a into a
compiler-builtins crate on top of the core crate and below the std crate.
This new crate still compiles the compiler-rt instrinsics using gcc-rs
but produces an .rlib instead of a static library.

Also, with this commit rustc no longer passes -lcompiler-rt to the
linker. This effectively makes the "no-compiler-rt" field of target
specifications a no-op. Users of `no_std` will have to explicitly add
the compiler-builtins crate to their crate dependency graph *if* they
need the compiler-rt intrinsics. Users of the `std` have to do nothing
extra as the std crate depends on compiler-builtins.

Finally, this a step towards lazy compilation of std with Cargo as the
compiler-rt intrinsics can now be built by Cargo instead of having to
be supplied by the user by some other method.

closes #34400

mk/clean.mk

@@ -102,7 +102,6 @@ define CLEAN_TARGET_STAGE_N
 clean$(1)_T_$(2)_H_$(3): \
 	    $$(foreach crate,$$(CRATES),clean$(1)_T_$(2)_H_$(3)-lib-$$(crate)) \
 	    $$(foreach tool,$$(TOOLS) $$(DEBUGGER_BIN_SCRIPTS_ALL),clean$(1)_T_$(2)_H_$(3)-tool-$$(tool))
-	$$(Q)rm -f $$(TLIB$(1)_T_$(2)_H_$(3))/libcompiler-rt.a
 	$(Q)rm -f $$(TLIB$(1)_T_$(2)_H_$(3))/librun_pass_stage* # For unix
 	$(Q)rm -f $$(TLIB$(1)_T_$(2)_H_$(3))/run_pass_stage* # For windows
 

mk/crates.mk

@@ -51,7 +51,7 @@
 
 TARGET_CRATES := libc std term \
                  getopts collections test rand \
-                 core alloc \
+                 compiler_builtins core alloc \
                  rustc_unicode rustc_bitflags \
 		 alloc_system alloc_jemalloc \
 		 panic_abort panic_unwind unwind
@@ -65,6 +65,7 @@ HOST_CRATES := syntax syntax_ext proc_macro syntax_pos $(RUSTC_CRATES) rustdoc f
 TOOLS := compiletest rustdoc rustc rustbook error_index_generator
 
 DEPS_core :=
+DEPS_compiler_builtins := core
 DEPS_alloc := core libc alloc_system
 DEPS_alloc_system := core libc
 DEPS_alloc_jemalloc := core libc native:jemalloc
@@ -77,12 +78,14 @@ DEPS_panic_abort := libc alloc
 DEPS_panic_unwind := libc alloc unwind
 DEPS_unwind := libc
 
+RUSTFLAGS_compiler_builtins := -lstatic=compiler-rt
+
 # FIXME(stage0): change this to just `RUSTFLAGS_panic_abort := ...`
 RUSTFLAGS1_panic_abort := -C panic=abort
 RUSTFLAGS2_panic_abort := -C panic=abort
 RUSTFLAGS3_panic_abort := -C panic=abort
 
-DEPS_std := core libc rand alloc collections rustc_unicode \
+DEPS_std := core libc rand alloc collections compiler_builtins rustc_unicode \
 	native:backtrace \
 	alloc_system panic_abort panic_unwind unwind
 DEPS_arena := std
@@ -153,6 +156,7 @@ TOOL_SOURCE_rustc := $(S)src/driver/driver.rs
 TOOL_SOURCE_rustbook := $(S)src/tools/rustbook/main.rs
 TOOL_SOURCE_error_index_generator := $(S)src/tools/error_index_generator/main.rs
 
+ONLY_RLIB_compiler_builtins := 1
 ONLY_RLIB_core := 1
 ONLY_RLIB_libc := 1
 ONLY_RLIB_alloc := 1

mk/main.mk

@@ -455,7 +455,10 @@ endif
 TSREQ$(1)_T_$(2)_H_$(3) = \
 	$$(HSREQ$(1)_H_$(3)) \
 	$$(foreach obj,$$(REQUIRED_OBJECTS_$(2)),\
-		$$(TLIB$(1)_T_$(2)_H_$(3))/$$(obj))
+		$$(TLIB$(1)_T_$(2)_H_$(3))/$$(obj)) \
+	$$(TLIB0_T_$(2)_H_$(3))/libcompiler-rt.a
+# ^ This copies `libcompiler-rt.a` to the stage0 sysroot
+# ^ TODO(stage0) update this to not copy `libcompiler-rt.a` to stage0
 
 # Prerequisites for a working stageN compiler and libraries, for a specific
 # target

mk/platform.mk

@@ -102,8 +102,6 @@ include $(wildcard $(CFG_SRC_DIR)mk/cfg/*.mk)
 define ADD_INSTALLED_OBJECTS
   INSTALLED_OBJECTS_$(1) += $$(CFG_INSTALLED_OBJECTS_$(1))
   REQUIRED_OBJECTS_$(1) += $$(CFG_THIRD_PARTY_OBJECTS_$(1))
-  INSTALLED_OBJECTS_$(1) += $$(call CFG_STATIC_LIB_NAME_$(1),compiler-rt)
-  REQUIRED_OBJECTS_$(1) += $$(call CFG_STATIC_LIB_NAME_$(1),compiler-rt)
 endef
 
 $(foreach target,$(CFG_TARGET), \