MIPS: properly stop .eh_frame generation

Commit 866b6a89c6 ("MIPS: Add DWARF unwinding to assembly") added
-fno-asynchronous-unwind-tables to KBUILD_CFLAGS to prevent compiler
from emitting .eh_frame symbols.
However, as MIPS heavily uses CFI, that's not enough. Use the
approach taken for x86 (as it also uses CFI) and explicitly put CFI
symbols into the .debug_frame section (except for VDSO).
This allows us to drop .eh_frame from DISCARDS as it's no longer
being generated.

Fixes: 866b6a89c6 ("MIPS: Add DWARF unwinding to assembly")
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/asm.h

@@ -20,10 +20,27 @@
 #include <asm/sgidefs.h>
 #include <asm/asm-eva.h>
 
+#ifndef __VDSO__
+/*
+ * Emit CFI data in .debug_frame sections, not .eh_frame sections.
+ * We don't do DWARF unwinding at runtime, so only the offline DWARF
+ * information is useful to anyone. Note we should change this if we
+ * ever decide to enable DWARF unwinding at runtime.
+ */
+#define CFI_SECTIONS	.cfi_sections .debug_frame
+#else
+ /*
+  * For the vDSO, emit both runtime unwind information and debug
+  * symbols for the .dbg file.
+  */
+#define CFI_SECTIONS
+#endif
+
 /*
  * LEAF - declare leaf routine
  */
 #define LEAF(symbol)					\
+		CFI_SECTIONS;				\
 		.globl	symbol;				\
 		.align	2;				\
 		.type	symbol, @function;		\
@@ -36,6 +53,7 @@ symbol:		.frame	sp, 0, ra;			\
  * NESTED - declare nested routine entry point
  */
 #define NESTED(symbol, framesize, rpc)			\
+		CFI_SECTIONS;				\
 		.globl	symbol;				\
 		.align	2;				\
 		.type	symbol, @function;		\

arch/mips/kernel/vmlinux.lds.S

@@ -224,6 +224,5 @@ SECTIONS
 		*(.options)
 		*(.pdr)
 		*(.reginfo)
-		*(.eh_frame)
 	}
 }