Save and restore floating point registers in get/set_mcontext().

[jhb]

Summary

arm64 and riscv were only saving and restoring floating point
registers for sendsig() and sys_sigreturn(), but not for getcontext(),
setcontext(), and swapcontext().

While here, remove a pointless check for uap being NULL from
sys_sigreturn().

Test Plan

• booted riscv64 under spike which should exercise sendsig/sigreturn
• have not tested arm64

[mhorne]

Just curious, why does spike in particular exercise sendsig/sigreturn?

[jhb]

In D23218#509338, @mhorne wrote:

Just curious, why does spike in particular exercise sendsig/sigreturn?

I'm just assuming that booting multiuser involves catching at least one signal somewhere along the way.