guix: remove usage of -Wl,-z,noexecstack for PPC64 HOST

The PPC64 ABI has a non-executable stack by default, and does not need a
GNU_STACK program header.

See also:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/include/asm/page_64.h#n92

Author: fanquake

contrib/guix/libexec/build.sh

@@ -249,10 +249,6 @@ case "$HOST" in
     *powerpc64*) HOST_LDFLAGS="${HOST_LDFLAGS} -Wl,--no-tls-get-addr-optimize" ;;
 esac
 
-case "$HOST" in
-    powerpc64-linux-*) HOST_LDFLAGS="${HOST_LDFLAGS} -Wl,-z,noexecstack" ;;
-esac
-
 # Make $HOST-specific native binaries from depends available in $PATH
 export PATH="${BASEPREFIX}/${HOST}/native/bin:${PATH}"
 mkdir -p "$DISTSRC"