build: move link flags to build.rs

Binary-specific link flags (-nostartfiles, -static, section stripping)
now use cargo:rustc-link-arg-bin so they don't break proc-macro or
build-script linking. ld-wrapper only strips sections from static
binaries.

scripts/ld-wrapper

@@ -2,20 +2,23 @@
 # Invoke mold, then strip junk sections from the output binary with objcopy.
 # This (more or less) removes sections that mold cannot discard itself, suck as:
 #  - .eh_frame / .eh_frame_hdr  - unwind tables from compiler_builtins
-#  - dynstr                     - mold emits this, even for fully static binaries
+#  - .dynstr                     - mold emits this, even for fully static binaries
 #  - .comment                   - compiler version string
 #
 #  We forward everything to mold via -fuse-ld, then post-process the output in place.
 
 set -eu
 
-# Locate the output file
+# Locate the output file and detect static linking
+IS_STATIC=0
 OUTPUT=""
 prev=""
 for arg in "$@"; do
+	case "$arg" in
+		-static) IS_STATIC=1 ;;
+	esac
 	if [ "$prev" = "-o" ]; then
 		OUTPUT="$arg"
-		break
 	fi
 	prev="$arg"
 done
@@ -23,8 +26,9 @@ done
 # Invoke mold via the cc driver, forward all original arguments
 cc -fuse-ld=mold "$@"
 
-# Remove sections that mold cannot discard
-if [ -n "$OUTPUT" ] && [ -f "$OUTPUT" ]; then
+# Only strip sections from fully static binaries.
+# Dynamic executables (i.e. build scripts, proc-macros) need .dynstr at runtime.
+if [ "$IS_STATIC" = 1 ] && [ -n "$OUTPUT" ] && [ -f "$OUTPUT" ]; then
 	objcopy \
 		--remove-section=.eh_frame \
 		--remove-section=.eh_frame_hdr \