gallivm: orcjit: use a mutex to protect symbol looking up

When a symbol is looked up for the first time, the associated function
is built, and the building process seems to be not thread-safe.

Use a mutex to protect the symbol looking up process, which should be
serialized when the function is not built, and fast when the function is
built.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>