[Z80] improve gas syntax #33

Job	Run time
build-llvm (macos-14, Release)	15m 50s
build-llvm (ubuntu-22.04, Release)	37m 35s
build-llvm (ubuntu-latest, Release)	39m 55s
build-llvm (macos-13, Release)	51m 3s
test-llvm-z80 (ubuntu-22.04, Release)	1m 20s
build-clang (macos-13, Release)	32m 52s
build-clang (macos-14, Release)	15m 2s
test-llvm-z80 (ubuntu-latest, Release)	1m 20s
build-clang (ubuntu-22.04, Release)	36m 12s
test-llvm-z80 (macos-13, Release)	2m 36s
test-llvm-z80 (macos-14, Release)	1m 30s
build-clang (ubuntu-latest, Release)	37m 35s
test-llvm	0s
nightly	27s
test-clang	0s
	4h 33m 17s

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