acl2: improve formula readability, use saved_acl2 scripts

[cho-m]

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Makefile already sets ACL2 at https://github.com/acl2/acl2/blob/master/GNUmakefile#L166-L169

PREFIXsaved_acl2 := ${PREFIX}saved_acl2${ACL2_SUFFIX}
PREFIXosaved_acl2 := ${PREFIX}osaved_acl2${ACL2_SUFFIX}

ACL2 ?= $(ACL2_WD)/${PREFIXsaved_acl2}

And setting ACL2_PAR handles to suffix at https://github.com/acl2/acl2/blob/master/GNUmakefile#L150-L151

ifdef ACL2_PAR
	ACL2_SUFFIX := $(ACL2_SUFFIX)p

write_env_script can also be used instead of writing out shell scripts.