Add recommendations for building linkers

[thesamesam]

We need a similar section to "What should you do when compiling compilers?" for linkers.

Noticed when looking at #622.

Some initial candidates for GNU Binutils:

• --disable-default-execstack
• --enable-error-execstack
• --enable-relro
• --enable-secureplt (for ppc)
• --enable-textrel-check=error
• --enable-separate-code / --enable-rosegment (although do see Consider -Wl,-z,separate-code for C and C++ Compiler Hardening Guide #588 (comment))

[david-a-wheeler]

+1, makes sense to me. Would you be willing to create a first draft pull request?

[thesamesam]

Happily, will do.