base: Fix ucontext compilation error for macOS
base: Fix ucontext compilation error for macOS

ucontext functions (like getcontext, setcontext etc) have been marked
as deprecated and are hence hidden in latest macOS releases.
This patch uncovers them; warnings wil be produced but compilation
won't fail since -Wno-error=deprecated-declarations flag is currently
used.

Change-Id: Ic10e6f77a38875828b1891eaed2f0626ecffff67
Signed-off-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11729
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu