Add support for LLD linker, cmake options for LLD and Gold

[vlstill]

A somewhat different approach to the same problem as in #3262, where @grg wrote:

LLD is much faster than gold in testing, so prefer LLD if it is found.

Test results for a debug build with the Intel Tofino backend after
touching a single file:

gold: 313.410 s (averaged across 10 runs)
lld: 151.783 s (averaged across 10 runs)

The difference is mainly in the options to enable/disable lookup for linkers.

Tested with GCC & Clang. Since the detection fails if the -fuse-ld=??? is not supported by the given gcc/clang version there is no need for version checks.

[grg]

Looks good to me.

It's slightly more verbose than my proposal, but it's better for at least two reasons:

1. It eliminates some duplication.
2. Total lines of code should be smaller, and with fewer levels of nested ifs, if we have 3+ linker options.

[davidbolvansky]

GCC LTO does not work with LLD.

(LLD does not support gcc lto files..)

[vlstill]

We could disable LLD if using static release & GCC. Although I would like to know why is LTO enable only with static builds, it seems like a rather ad-hoc decision to me.