RISC-V: Document the degree of position independence that medany affords

The code generated by -mcmodel=medany is defined to be
position-independent, but is not guaranteed to function correctly when
linked into position-independent executables or libraries.  See the
recent discussion at the psABI specification [1] for more details.

It would be better to reject these invalid sequences when linking, but
as pointed out in a recent LD bug [2] there may be some compatibility
issues related to the PCREL_HI20 relocations used to initialize GP.
Given the complexity here it's unlikely we'll be able to reject these
sequences any time soon, so instead just document that these may not
work.

[1]: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/245
[2]: https://sourceware.org/bugzilla/show_bug.cgi?id=28789

gcc/ChangeLog:

	* doc/invoke.texi (RISC-V -mcmodel=medany): Document the degree
	of position independence that -mcmodel=medany affords.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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