Support `$LLVM_VERSION`, `$LLVM_TARGETS`, and `$LLVM_LDFLAGS` #12679

Job	Run time
x86_64-gnu-test (1.7.3)	34m 55s
x86_64-gnu-test (1.8.2)	34m 7s
x86_64-gnu-test (1.9.2)	35m 47s
x86_64-gnu-test (1.10.1)	35m 5s
x86_64-gnu-test (1.11.2)	34m 9s
x86_64-gnu-test (1.12.2)	34m 54s
x86_64-gnu-test (1.13.3)	33m 4s
x86_64-gnu-test (1.14.0)	36m 34s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	31m 49s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 43s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	36m 2s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 20s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	32m 50s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 48s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	34m 12s
check_format	1m 32s
x86_64-gnu-test-preview_mt	5m 31s
x86_64-musl-test	38m 41s
	9h 23m 3s

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