x86/alternatives: Do not use integer constant suffixes in inline asm

The usage of the BIT() macro in inline asm code was introduced in 6.3 by
the commit in the Fixes tag. However, this macro uses "1UL" for integer
constant suffixes in its shift operation, while gas before 2.28 does not
support the "L" suffix after a number, and gas before 2.27 does not
support the "U" suffix, resulting in build errors such as the following
with such versions:

  ./arch/x86/include/asm/uaccess_64.h:124: Error: found 'L', expected: ')'
  ./arch/x86/include/asm/uaccess_64.h:124: Error: junk at end of line,
  first unrecognized character is `L'

However, the currently minimal binutils version the kernel supports is
2.25.

There's a single use of this macro here, revert to (1 << 0) that works
with such older binutils.

As an additional info, the binutils PRs which add support for those
suffixes are:

  https://sourceware.org/bugzilla/show_bug.cgi?id=19910
  https://sourceware.org/bugzilla/show_bug.cgi?id=20732

  [ bp: Massage and extend commit message. ]

Fixes: 5d1dd96 ("x86/alternatives: Add alt_instr.flags")
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Link: https://lore.kernel.org/lkml/a9aae568-3046-306c-bd71-92c1fc8eeddc@linux.alibaba.com/

arch/x86/include/asm/alternative.h

@@ -8,7 +8,7 @@
 
 #define ALT_FLAGS_SHIFT		16
 
-#define ALT_FLAG_NOT		BIT(0)
+#define ALT_FLAG_NOT		(1 << 0)
 #define ALT_NOT(feature)	((ALT_FLAG_NOT << ALT_FLAGS_SHIFT) | (feature))
 
 #ifndef __ASSEMBLY__