As of r360984, LLD supports dependent libraries feature for ELF.
libunwind and libc++ have library dependencies: libdl and libpthread
respectively, which means that when libunwind and libc++ are being
statically linked (using -static-libstdc++ flag), user has to manually
specify -ldl -lpthread which is onerous. This change includes the
lib pragma to specify the library dependencies directly in the source
that uses those libraries. This doesn't make any difference when using
linkers that don't support dependent libraries. However, when using
LLD that has dependent libraries feature, users no longer have to
manually specifying library dependencies when using static linking,
linker will pick them up automatically. We only support this feature
on Linux for now, additional platforms can be added in the future.