x86, threadinfo: Redo "x86: Use inline assembler to get sp"

This patch restores the changes of commit dff38e3e93 "x86: Use inline
assembler instead of global register variable to get sp". They got lost
in commit 198d208df4 "x86: Keep thread_info on thread stack in x86_32"
while moving the code to arch/x86/kernel/irq_32.c.

Quoting Andi from commit dff38e3e93:

"""
LTO in gcc 4.6/47. has trouble with global register variables. They were
used to read the stack pointer. Use a simple inline assembler statement
with a mov instead.

This also helps LLVM/clang, which does not support global register
variables.
"""

Cc: Steven Rostedt <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Andi Kleen <[email protected]>
Signed-off-by: Mathias Krause <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: H. Peter Anvin <[email protected]>

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