linux: don't assume vdso exists
linux: don't assume vdso exists

AT_SYSINFO_EHDR may be defined even if no vdso is used. e.g. 32-bit ARM
processes have a vdso on 64-bit ARM cpus, but not on 32-bit cpus.

Change-Id: I4d9ce029bb47efc33ea16cb4c5c2055c1b9330c9
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/1860935
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>
Commit-Queue: Joshua Peraza <f5809a5e35ec7e1bd0e25cb0a0b68bb433cf6f17@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad