CaptureContext isn't actually used on Fuchsia and there is a desire to
remove `ucontext_t` from Fuchsia as it isn't a real concept on Fuchsia
and was only added as a placeholder. Moreover, `ucontext_t` won't ever
be added to Fuchsia for RISC-V.
Bug: fuchsia:123052
Fixed: fuchsia:131112
Fixed: fuchsia:127655
Tested: `fx test crashpad` on core.x64 emulator
Tested: `fx test crashpad` on ARM64 device
Tested: `fx shell crasher` @ 16b19a9891978487 on ARM64 device, ran
through Breakpad stackwalker locally as well
Tested: `fx build crashpad_tests` for minimal.riscv64
Change-Id: I4695054426df78a9deff8c9ea9c478b5bf9701b1
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/4717085
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Thomas Gales <tgales@google.com>
8dcf2b216f