CI: test PPC32 big endian target using `cross` (#73) #101

Job	Run time
doc	1m 12s
rustfmt	3s
no_std (1.75.0, armv7a-none-eabi)	33s
clippy	19s
no_std (1.75.0, thumbv7em-none-eabi)	33s
cross (powerpc-unknown-linux-gnu, 1.75.0)	1m 2s
test (1.75.0)	35s
no_std (1.75.0, wasm32-unknown-unknown)	37s
cross (powerpc-unknown-linux-gnu, stable)	54s
test (stable)	39s
no_std (stable, armv7a-none-eabi)	26s
no_std (stable, thumbv7em-none-eabi)	26s
no_std (stable, wasm32-unknown-unknown)	26s
	7m 45s

Provide feedback