Remove set -x from gcc wrappers and fix behavior with 0 arguments (#1728)

Fixes #1618.

Additionally fixes the behavior when invoked with 0 arguments:
`${@}` is a single word, regardless of how many arguments were passed to
the program. To expand it as an array, it must be `$@`.
See
https://www.gnu.org/software/bash/manual/html_node/Shell-Expansions.html.

```
docker/musl-gcc.sh: line 73: @: unbound variable
docker/freebsd-gcc.sh: line 16: @: unbound variable
```

Author: Emilgardis

docker/freebsd-gcc.sh

@@ -3,15 +3,14 @@
 # the freebsd images need libstdc++ to be linked as well
 # otherwise, we get `undefined reference to `std::ios_base::Init::Init()'`
 
-set -x
 set -euo pipefail
 
 main() {
     if [[ $# -eq 0 ]]; then
-        exec "${CROSS_TOOLCHAIN_PREFIX}gcc" "${@}"
+        exec "${CROSS_TOOLCHAIN_PREFIX}gcc"
     else
         exec "${CROSS_TOOLCHAIN_PREFIX}gcc" "${@}" -lc++ -lstdc++
     fi
 }
 
-main "${@}"
+main "$@"

docker/musl-gcc.sh

@@ -19,7 +19,6 @@
 #   missing soft-fp routine `__trunctfsf2`
 #   https://github.com/rust-lang/compiler-builtins/pull/483
 
-set -x
 set -euo pipefail
 
 main() {
@@ -28,9 +27,9 @@ main() {
     minor=$(rustc_minor_version)
 
     if [[ $# -eq 0 ]] || [[ "${minor}" -ge "${patched_minor}" ]]; then
-        exec "${CROSS_TOOLCHAIN_PREFIX}"gcc "${@}"
+        exec "${CROSS_TOOLCHAIN_PREFIX}"gcc "$@"
     else
-        exec "${CROSS_TOOLCHAIN_PREFIX}"gcc "${@}" -lgcc -static-libgcc
+        exec "${CROSS_TOOLCHAIN_PREFIX}"gcc "$@" -lgcc -static-libgcc
     fi
 }
 
@@ -71,4 +70,4 @@ rustc_patch_version() {
     echo "${CROSS_RUSTC_PATCH_VERSION}"
 }
 
-main "${@}"
+main "$@"