[Object][Archive] Support a new archive member /<ECSYMBOLS>/

Some libraries (e.g., arm64rt.lib) from the Windows WDK (version 10.0.22000.0)
contain an undocumented special member '/<ECSYMBOLS>/'. This causes llvm-lib to
fail with the following error:

"truncated or malformed archive (long name offset characters after the '/' are
not all decimal numbers: '<ECSYMBOLS>/' for archive member header at offset 162)"

The '/<ECSYMBOLS>/' member does not seem to be documented anywhere, but might be
related to the ARM64EC ABI Microsoft announced last year.

https://blogs.windows.com/windowsdeveloper/2021/06/28/announcing-arm64ec-building-native-and-interoperable-apps-for-windows-11-on-arm/

Reviewed By: thieta, thakis

Differential Revision: https://reviews.llvm.org/D127135