Correctly find custom-built `llc` #2717

Job	Run time
Validate Windows Docker image builds	3m 9s
Validate GNU Docker image builds	36s
arm Linux glibc	49m 17s
riscv64 Linux glibc	49m 16s
Lint bash, docker, markdown, and yaml	1m 44s
Validate GNU Docker release image builds	27s
Validate musl Docker image builds	19s
Validate musl Docker release image builds	19s
Validate Windows Docker release image builds	1m 16s
Verify CHANGELOG is valid	10s
x86-64 Linux glibc	20m 59s
x86-64 Linux musl	21m 52s
aarch64 Linux glibc	21m 14s
arm64 Apple Darwin	21m 8s
armhf Linux glibc	49m 11s
x86-64 Apple Darwin	37m 57s
x86-64 Windows MSVC	37m 18s
use dtrace	20m 41s
use pool_memalign	23m 24s
use runtimestats	22m 25s
	6h 22m 42s

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