disable build/install of temp sysroot headers for native compilers

these are not needed since the cross toolchain being used to build the
native compiler already has its own libc and headers.

but more importantly, due to lack of certain dependency chains that
would be present for a cross compiler build, it was possible for two
invocations of musl's make process to occur in parallel (one for temp
sysroot headers, one for actual libc build). this could (and was
observed to) lead to a race condition where alltypes.h was momentarily
truncated in the middle of the libc build.

litecross/Makefile

@@ -63,7 +63,7 @@ SYSROOT = /$(TARGET)
 FULL_MUSL_CONFIG += CC="$(XGCC)" LIBCC="../obj_toolchain/$(TARGET)/libgcc/libgcc.a" 
 MUSL_VARS = AR=../obj_toolchain/binutils/ar RANLIB=../obj_toolchain/binutils/ranlib
 obj_musl/.lc_built: | obj_toolchain/$(TARGET)/libgcc/libgcc.a
-obj_toolchain/.lc_built: | obj_sysroot/.lc_libs
+obj_toolchain/.lc_built: | obj_sysroot/.lc_libs obj_sysroot/.lc_headers
 else
 SYSROOT = /
 FULL_TOOLCHAIN_CONFIG += --host=$(TARGET)
@@ -177,7 +177,7 @@ obj_sysroot/.lc_libs: | obj_musl/.lc_built
 	cd obj_musl && $(MAKE) $(MUSL_VARS) DESTDIR=$(PWD)/obj_sysroot install
 	touch $@
 
-obj_toolchain/.lc_built: | obj_toolchain/.lc_configured obj_sysroot/.lc_headers
+obj_toolchain/.lc_built: | obj_toolchain/.lc_configured
 	cd obj_toolchain && $(MAKE) MAKE="$(MAKE)" LC_ROOT=$(PWD)
 	touch $@