gdb: Implement amd64 linux shadow stack support for inferior calls.

gdb: Implement amd64 linux shadow stack support for inferior calls.

This patch enables inferior calls to support Intel's Control-Flow
Enforcement Technology (CET), which provides the shadow stack feature
for the x86 architecture.
Following the restriction of the linux kernel, enable inferior calls
for amd64 only.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>