cc-wrapper: Add `set -x` tracing for NIX_DEBUG >= 7

pkgs/build-support/cc-wrapper/cc-wrapper.sh

···
 set -eu -o pipefail
 shopt -s nullglob
 
+if (( "${NIX_DEBUG:-0}" >= 7 )); then
+    set -x
+fi
+
 path_backup="$PATH"
 
 # That @-vars are substituted separately from bash evaluation makes

pkgs/build-support/cc-wrapper/gnat-wrapper.sh

···
 set -eu -o pipefail
 shopt -s nullglob
 
+if (( "${NIX_DEBUG:-0}" >= 7 )); then
+    set -x
+fi
+
 # N.B. Gnat is not used during bootstrapping, so we don't need to
 # worry about the old bash empty array `set -u` workarounds.
 

pkgs/build-support/cc-wrapper/gnatlink-wrapper.sh

···
 set -eu -o pipefail
 shopt -s nullglob
 
+if (( "${NIX_DEBUG:-0}" >= 7 )); then
+    set -x
+fi
+
 # N.B. Gnat is not used during bootstrapping, so we don't need to
 # worry about the old bash empty array `set -u` workarounds.
 

pkgs/build-support/cc-wrapper/ld-solaris-wrapper.sh

···
 set -eu -o pipefail
 shopt -s nullglob
 
+if (( "${NIX_DEBUG:-0}" >= 7 )); then
+    set -x
+fi
+
 declare -a args=("$@")
 # I've also tried adding -z direct and -z lazyload, but it gave too many problems with C++ exceptions :'(
 # Also made sure libgcc would not be lazy-loaded, as suggested here: https://www.illumos.org/issues/2534#note-3

pkgs/build-support/cc-wrapper/ld-wrapper.sh

···
 set -eu -o pipefail
 shopt -s nullglob
 
+if (( "${NIX_DEBUG:-0}" >= 7 )); then
+    set -x
+fi
+
 path_backup="$PATH"
 
 # phase separation makes this look useless