[LLD][COFF] Use getMachineArchType in LinkerDriver::getArch. (#87499)

Adds support for ARM64EC, which should use the same search paths as
ARM64.

It's similar to #87370 and #87495. The test is based on the existing x86
test. Generally ARM64EC libraries are shipped together with native ARM64
libraries (using ECSYMBOLS section mechanism).

getMachineArchType uses Triple::thumb, while the existing
implementation uses Triple::arm. It's ultimately passed to
MSVCPaths.cpp functions, so modify them to accept both forms.