Statically emit ELF notes for Swift's metadata sections.
ELF does not preserve section information like Mach-O and COFF do. This PR adds
additional exported symbols on platforms that use ELF to help us find Swift's
metadata sections at runtime. These symbols are emitted as [ELF notes](https://man7.org/linux/man-pages/man5/elf.5.html),
which are a kind of optional metadata that can be iterated at runtime using the
[`dl_iterate_phdr()`](https://man7.org/linux/man-pages/man3/dl_iterate_phdr.3.html)
function.
Although the ELF standard says that notes can be stripped without affecting a
program's behavior, in practice they are used widely for similar purposes e.g.
by GCC/ld and various OS vendors.
A more permanent solution may involve teaching llvm and lld about some new
section name or attribute that translates to a specific custom ELF program
header in the `PT_LOUSER`–`PT_HIUSER` range, then emitting that information for
all of Swift's standard sections as well as anything marked `@_section`.
Resolves #76698.
