PKG-211: Link LLVM statically

[dutow]

This commit modifies the build scripts so that llvmjit.so links LLVM statically instead of dynamically. No other binary affected, the change shouldn't be user visible at all.

The memory footprint of postgres is also unaffected: only the llvmjit.so dynamic library is affected. If postgres doesn't use JIT, the LLVM library isn't loaded, as before. If it uses JIT, it is loaded as part of llvmjit.so, sa before.

The only change is the binary size of llvmjit.so.

ldd lib/postgresql/llvmjit.so
	linux-vdso.so.1 (0x00007fff3fd2c000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007fc6596bc000)
	libtinfo.so.6 => /lib/x86_64-linux-gnu/libtinfo.so.6 (0x00007fc65968a000)
	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fc65945e000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fc659377000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fc659357000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fc65912c000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fc65c078000)