Replace __riscv__ with __riscv.

__riscv__ is not pre-defined anymore by latest version of GNU compiler.

Sponsored by: DARPA, AFRL

Update limits on makecontext() arguments in the setcontext_link test.

sparc64 and riscv do not support 10 arguments, but MIPS now does.
While here, combine clauses for architectures that support the same
number of arguments to reduce duplication.

Sponsored by: DARPA / AFRL

Add #else case in run(..) to fix test on non-{amd64,arm64,mips} after
recent refactoring to the test

Add back #else block accidentally removed in r312145

This fixes the build with !(amd64|arm64|mips*)

Try to consolidate the #ifdef soup into FreeBSD vs non-FreeBSD blocks better

Allow up to 6 arguments only on MIPS.

Sponsored by: DARPA, AFRL
Sponsored by: HEIF5

Pass 8 arguments to makecontext on arm64 as this is all we support.

Obtained from: EuroBSDCon Devsummit
Sponsored by: ABT Systems Ltd

Only pass 6 arguments to the 'run' function on amd64. amd64's
makecontext on FreeBSD only supports a maximum of 6 arguments. This
fixes the setcontext_link test on amd64.

PR: 194828

Expect lib.libc.sys.getcontext_test.setcontext_link to fail on amd64; add
additional debugging to make the underlying problem more visible

Calling setcontext(2) on amd64 as shown in the test program is failing on
amd64, not i386, with a return code of -1 and an errno of EINVAL

Further investigation is being done in the PR to determine the root cause for
the failure

PR: 194828
Tested with the following configuration:
- amd64/i386
- 11.0-CURRENT @ r273153
- 100 times in a tight loop as root with the following commands...
-- kyua test lib/libc
-- kyua test lib/libc/sys
-- kyua test lib/libc/sys/getcontext_test