Bug 1747757 - Remove -l from OS_LIBS set from gyp. r=firefox-build-system-reviewers,mhentges

NSS gyp files use -l$lib, and while OS_LIBS accepts this form and passes
it through, it's not actually a recognized way to link libraries on
clang-cl builds. So, re-normalize the values to not include -l, which
will add it back in the backend when appropriate, or switch to $lib.lib
on platforms that use this form.

Differential Revision: https://phabricator.services.mozilla.com/D134736

python/mozbuild/mozbuild/frontend/gyp_reader.py

@@ -203,6 +203,10 @@ def process_gyp_result(
         os_libs = []
         for l in libs:
             if l.startswith("-"):
+                if l.startswith("-l"):
+                    # Remove "-l" for consumption in OS_LIBS. Other flags
+                    # are passed through unchanged.
+                    l = l[2:]
                 if l not in os_libs:
                     os_libs.append(l)
             elif l.endswith(".lib"):