mach: Fix cross-compiling from windows to non windows (#36070)

Since we switched to using `aws-lc-rs` instead of `ring`,
cross-compiling on windows (to non-windows) had been broken. Using the
default MSVC Generator results in CMake erroneously being configured for
the host platform, with failing compiler checks. Switching to Ninja
fixes that issue.

The CMake build rules of aws-lc-rs also make use of `tr` and assume it
is installed, hence we provide a helpful error message suggesting to try
using mach from `git bash` which does provide the `tr` command.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix cross-compiling servo on windows hosts to
non-windows targets
- [x] These changes do not require tests because: We don't test
cross-compiling from windows hosts in CI. These changes were manually
tested by compiling for OpenHarmony on windows.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

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