MIPS: Fix odd fp register warnings with MIPS64r2

Building 32-bit MIPS64r2 kernels produces warnings like the following
on certain toolchains (such as GNU assembler 2.24.90, but not GNU
assembler 2.28.51) since commit 22b8ba765a72 ("MIPS: Fix MIPS64 FP
save/restore on 32-bit kernels"), due to the exposure of fpu_save_16odd
from fpu_save_double and fpu_restore_16odd from fpu_restore_double:

arch/mips/kernel/r4k_fpu.S:47: Warning: float register should be even, was 1
...
arch/mips/kernel/r4k_fpu.S:59: Warning: float register should be even, was 1
...

This appears to be because .set mips64r2 does not change the FPU ABI to
64-bit when -march=mips64r2 (or e.g. -march=xlp) is provided on the
command line on that toolchain, from the default FPU ABI of 32-bit due
to the -mabi=32. This makes access to the odd FPU registers invalid.

Fix by explicitly changing the FPU ABI with .set fp=64 directives in
fpu_save_16odd and fpu_restore_16odd, and moving the undefine of fp up
in asmmacro.h so fp doesn't turn into $30.

Fixes: 22b8ba765a72 ("MIPS: Fix MIPS64 FP save/restore on 32-bit kernels")
Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.0+: 22b8ba765a72: MIPS: Fix MIPS64 FP save/restore on 32-bit kernels
Cc: <stable@vger.kernel.org> # 4.0+
Patchwork: https://patchwork.linux-mips.org/patch/17656/

arch/mips/include/asm/asmmacro.h

@@ -19,6 +19,9 @@
 #include <asm/asmmacro-64.h>
 #endif
 
+/* preprocessor replaces the fp in ".set fp=64" with $30 otherwise */
+#undef fp
+
 /*
  * Helper macros for generating raw instruction encodings.
  */
@@ -105,6 +108,7 @@
 	.macro	fpu_save_16odd thread
 	.set	push
 	.set	mips64r2
+	.set	fp=64
 	SET_HARDFLOAT
 	sdc1	$f1,  THREAD_FPR1(\thread)
 	sdc1	$f3,  THREAD_FPR3(\thread)
@@ -163,6 +167,7 @@
 	.macro	fpu_restore_16odd thread
 	.set	push
 	.set	mips64r2
+	.set	fp=64
 	SET_HARDFLOAT
 	ldc1	$f1,  THREAD_FPR1(\thread)
 	ldc1	$f3,  THREAD_FPR3(\thread)
@@ -234,9 +239,6 @@
 	.endm
 
 #ifdef TOOLCHAIN_SUPPORTS_MSA
-/* preprocessor replaces the fp in ".set fp=64" with $30 otherwise */
-#undef fp
-
 	.macro	_cfcmsa	rd, cs
 	.set	push
 	.set	mips32r2