x86: use inline asm to retrieve stack pointer

According to GCC's documentation, the only supported use for specifying
registers for local variables is "to specify registers for input and
output operands when calling Extended asm." Using it as a shortcut to
get the value in a register isn't guaranteed to work, and clang
complains that the variable is uninitialized.

Signed-off-by: Bill Wendling <morbo@google.com>
Message-Id: <20191030210419.213407-7-morbo@google.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>