x86/entry: Merge thunk_64.S and thunk_32.S into thunk.S

The code in thunk_64.S and thunk_32.S are exactly the same except for
the comments. Merge them in to thunk.S.

And since thunk_32.S was originated from thunk_64.S, the new merged
thunk.S is actually renamed from thunk_64.S.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240407090558.3395-9-jiangshanlai@gmail.com

Author: Lai Jiangshan

arch/x86/entry/Makefile

@@ -17,7 +17,7 @@ obj-y				+= common.o
 obj-y				+= vdso/
 obj-y				+= vsyscall/
 
-obj-$(CONFIG_PREEMPTION)	+= thunk_$(BITS).o
+obj-$(CONFIG_PREEMPTION)	+= thunk.o
 CFLAGS_entry_fred.o		+= -fno-stack-protector
 CFLAGS_REMOVE_entry_fred.o	+= -pg $(CC_FLAGS_FTRACE)
 obj-$(CONFIG_X86_FRED)		+= entry_64_fred.o entry_fred.o