avutil/x86/asm: Remove wrong comment, rename FF_REG_sp

Before FFmpeg commit 531b0a316b24f00965cd8a88efdbea2c6d63147f,
FFmpeg used REG_SP as macro for the stack pointer, yet this
clashed with a REG_SP define in Solaris system headers, so it
was changed to REG_sp and a comment was added for this.

Libav fixed it by adding an FF_ prefix to the macros in
1e9c5bf4c136fe9e010cc8a7e7270bba0d1bf45e. FFmpeg switched
to using these prefixes in 9eb3da2f9942cf1b1148d242bccfc383f666feb6,
using FF_REG_sp instead of Libav's FF_REG_SP. In said commit
the comment was changed to claim that Solaris system headers
define FF_REG_SP, but this is (most likely) wrong.

This commit removes the wrong comment and renames the (actually unused)
macro to FF_REG_SP to make it consistent with FF_REG_BP.

Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/x86/asm.h

@@ -38,8 +38,7 @@ typedef struct ymm_reg { uint64_t a, b, c, d; } ymm_reg;
 #    define FF_PTR_SIZE "8"
 typedef int64_t x86_reg;
 
-/* FF_REG_SP is defined in Solaris sys headers, so use FF_REG_sp */
-#    define FF_REG_sp "rsp"
+#    define FF_REG_SP "rsp"
 #    define FF_REG_BP "rbp"
 #    define FF_REGBP   rbp
 #    define FF_REGa    rax
@@ -60,7 +59,7 @@ typedef int64_t x86_reg;
 #    define FF_PTR_SIZE "4"
 typedef int32_t x86_reg;
 
-#    define FF_REG_sp "esp"
+#    define FF_REG_SP "esp"
 #    define FF_REG_BP "ebp"
 #    define FF_REGBP   ebp
 #    define FF_REGa    eax