powerpc/boot: Fix dash warning

'commit b2accfe ("powerpc/boot: Check for ld-option support")' suppressed
linker warnings, but the expressed used did not go well with POSIX shell (dash)
resulting with this warning

arch/powerpc/boot/wrapper: 237: [: 0: unexpected operator
ld: warning: arch/powerpc/boot/zImage.epapr has a LOAD segment with RWX permissions

Fix the check to handle the reported warning. Patch also fixes
couple of shellcheck reported errors for the same line.

In arch/powerpc/boot/wrapper line 237:
if [ $(${CROSS}ld -v --no-warn-rwx-segments &>/dev/null; echo $?) -eq 0 ]; then
     ^-- SC2046 (warning): Quote this to prevent word splitting.
       ^------^ SC2086 (info): Double quote to prevent globbing and word splitting.
                                            ^---------^ SC3020 (warning): In POSIX sh, &> is undefined.

Fixes: b2accfe ("powerpc/boot: Check for ld-option support")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250423082154.30625-1-maddy@linux.ibm.com

arch/powerpc/boot/wrapper

@@ -234,7 +234,7 @@ fi
 
 # suppress some warnings in recent ld versions
 nowarn="-z noexecstack"
-if [ $(${CROSS}ld -v --no-warn-rwx-segments &>/dev/null; echo $?) -eq 0 ]; then
+if "${CROSS}ld" -v --no-warn-rwx-segments >/dev/null 2>&1; then
 	nowarn="$nowarn --no-warn-rwx-segments"
 fi