configure: Fix flags handling for pb little-endian targets #263

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	29m 51s
ci (a6osx, macos-12)	20m 34s
ci (ta6osx, macos-12)	25m 39s
ci (arm64osx, macos-14)	8m 13s
ci (tarm64osx, macos-14)	10m 11s
ci (i3le, ubuntu-22.04)	11m 34s
ci (ti3le, ubuntu-22.04)	14m 42s
ci (a6le, ubuntu-22.04)	8m 52s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	11m 47s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	10m 25s
ci (i3nt, windows-2022, vs)	11m 34s
ci (ti3nt, windows-2022, vs)	14m 25s
ci (a6nt, windows-2022, vs)	11m 53s
ci (ta6nt, windows-2022, vs)	15m 13s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 29s
	3h 38m 22s

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