build-system/gnu: Turn #:phases into a gexp when cross-compiling.

build-system/gnu: Turn #:phases into a gexp when cross-compiling.

Previously, we'd get this error:

  $ guix build --target=aarch64-linux-gnu r-minimal -d --no-grafts
  guix build: error: path ‘/gnu/store/[^-]+-glibc-[^-]+-static’ is not valid

This is because the sexp would be passed as an input of the surrounding
gexp in 'gnu-cross-build', and thus
"/gnu/store/[^-]+-glibc-[^-]+-static" would be interpreted as a source
file name, as in this example:

  scheme@(guix gexp)> #~(foo #$(list 'whatever "/gnu/store/[^-]+-glibc-[^-]+-static"))
  $11 = #<gexp (foo #<gexp-input (whatever "/gnu/store/[^-]+-glibc-[^-]+-static"):out>) 7f098badec30>
  scheme@(guix gexp)> (gexp-inputs $11)
  $12 = (#<gexp-input "/gnu/store/[^-]+-glibc-[^-]+-static":out>)

Fixes <https://issues.guix.gnu.org/59073>.
Reported by Christopher Baines <mail@cbaines.net>.

* guix/build-system/gnu.scm (gnu-cross-build): When PHASES is a pair,
pass it through 'sexp->gexp'.