Customize output section descriptions without specifying the output section order

[Fangrui Song]

Hi, I filed https://sourceware.org/bugzilla/show_bug.cgi?id=26404 last year
about a feature customizing output section descriptions without specifying the
output section order, which is a generic answer to various ad-hoc features
desired by linker users.

Do you have an opinion on whether this can be used implicitly? Being implicit
means that the linker script can be used in a way similar to glibc
libc.so/libm.a (ld ... -lc -lm can pick up such linker scripts disguised as .so
and .a files.) and does not require an option like
-T/--script/--overwrite-section-script.

https://sourceware.org/bugzilla/show_bug.cgi?id=26404#c10
https://reviews.llvm.org/D103303#2797857 are arguments preferring implicit
usage.