Reland "Reland "[llvm-exegesis] Switch from MCJIT to LLJIT (#72838)"

This reverts commit ea5de602.

This patch was reverted as it broke a test on x86_64 Darwin due to the
symbol naming being different between platforms. As Darwin isn't a
supported platform for executing snippets for llvm-exegesis, we can just
disable the test to fix the issue.