Update vendor/libarchive to 3.7.6

Security fixes:
#2330 iso: be more cautious about parsing ISO-9660 timestamps
#2343 tar: clean up linkpath between entries

Important bugfixes:
#2338 tar: fix memory leaks when processing symlinks or parsing pax headers

Obtained from: libarchive
Vendor commit: f0a0bc6b3046e34c9d6981f8c026da51fea12c89